Fix template trampoline overload lookup failure

Problem
=======

The template trampoline pattern documented in PR #322 has a problem with
virtual method overloads in intermediate classes in the inheritance
chain between the trampoline class and the base class.

For example, consider the following inheritance structure, where `B` is
the actual class, `PyB<B>` is the trampoline class, and `PyA<B>` is an
intermediate class adding A's methods into the trampoline:

    PyB<B> -> PyA<B> -> B -> A

Suppose PyA<B> has a method `some_method()` with a PYBIND11_OVERLOAD in
it to overload the virtual `A::some_method()`.  If a Python class `C` is
defined that inherits from the pybind11-registered `B` and tries to
provide an overriding `some_method()`, the PYBIND11_OVERLOADs declared
in PyA<B> fails to find this overloaded method, and thus never invoke it
(or, if pure virtual and not overridden in PyB<B>, raises an exception).

This happens because the base (internal) `PYBIND11_OVERLOAD_INT` macro
simply calls `get_overload(this, name)`; `get_overload()` then uses the
inferred type of `this` to do a type lookup in `registered_types_cpp`.
This is where it fails: `this` will be a `PyA<B> *`, but `PyA<B>` is
neither the base type (`B`) nor the trampoline type (`PyB<B>`).  As a
result, the overload fails and we get a failed overload lookup.

The fix
=======

The fix is relatively simple: we can cast `this` passed to
`get_overload()` to a `const B *`, which lets get_overload look up the
correct class.  Since trampoline classes should be derived from `B`
classes anyway, this cast should be perfectly safe.

This does require adding the class name as an argument to the
PYBIND11_OVERLOAD_INT macro, but leaves the public macro signatures
unchanged.

Doc fix for OVERLOAD*_NAME macros

The documentation says the string-valued python function name goes
after the C++ function, but it actually goes before it.

Check for style issues during docs build

Minor doc fix: ``make test`` -> ``make pytest``

The test target changed with the new testing framework.

This adds a tool that checks style (currently just for tabs instead of
spaces in files under include/tests/docs) and produces a travis-ci build
failure if any problems are found.

Fix module file name when working with debug builds of Python

Added support for exposing classes with private destructors and corresponding documentation

Fixes #365. `sysconfig.get_config_var('SO')` already returns the correct
PYTHON_MODULE_EXTENSION, even for debug builds, so there is no need to
add anything else manually.

Don't install pytest from cmake, just fail instead

Installing something outside the project directory from a cmake
invocation is overly intrusive; this changes tests/CMakeLists.txt to
just fail with an informative message instead, and changes the
travis-ci builds to install pytest via pip or apt-get.

Test absence of optional dependencies and CMake automatic discovery functions

ccache on Travis was never configured properly so the setting never
actually did anything. Enabling ccache for real brings other issues:
due to the way the preprocessor is handled, some of the Python header
macros produce bogus compiler warnings (which in turn produce errors
with -Werror). ccache also requires additional configuration on OS X
and docker. It would reduce compile time by ~30 seconds at best, so
it's not worth the trouble.

[skip appveyor]

This build makes sure everything still works without optional
dependencies (numpy/scipy/eigen) and also tests the automatic
discovery functions in CMake (Python version, C++ standard).

[skip appveyor]

Fix test suite failure without numpy and improve module init diagnostics

Fix dtype::strip_padding() on Intel compiler

Fixes #357.

Add support for iterators with different begin/end types

- ICPC can't handle the NCVirt trampoline which returns a non-copyable
  type, which is likely due to a constexpr/SFINAE issue. This disables
  the test on that compiler so that at least the rest can be tested.

Fix int_ shadowing problem in detail namespace

If operators.h is included, int_ function in the `detail`
namespace will shadow pybind11::int_ type, so the fully qualified
name has to be used.

Workaround for random failure of pytest capture on Windows

pytest can capture test output both globally (controlled by the cmd line
flag --capture) or locally (`capsys` and `capfd` fixtures). Enabling both
methods at the same time causes problems on Windows: test output is not
captured sometimes, resulting in test failure. This happens seemingly at
random.

This workaround disables global output capture ("-s", i.e. "--capture=no")
leaving only the local capture fixtures. As a side-effect test output on
AppVeyor CI is a little messy, but this will have to do until a better
solution is found.

Port test suite to pytest