Support keep_alive where nurse may be None

Build docs just once, in a dedicated build

For example keep_alive<0,1>() should work where the return value may sometimes be None. At present a "Could not allocate weak reference!" exception is thrown.
Update documentation to clarify behaviour of keep_alive when nurse is None or does not support weak references.

Rather than adding an `if [ -n "$DOCS" ]` as a separate install
instruction, this simplifies the travis-ci logic to do the pip/venv
setup in the `before_install` hook, leaving the install hook to just
install the needed packages.

This makes the default install script simpler: it doesn't need to check
NATIVE_DEPS or DOCS because both of those now override `install`
anyway, so the top-level `install` is really just the install for the
gcc-4.8 and osx builds; the docker builds and docs build override
install completely.

This switches the travis-ci build to build the docs in a dedicated
build rather than as part of the regular linux/osx builds.

Build sphinx docs (with -W) during travis-ci builds

Fix sphinx doc missing code blocks and warnings

The missing empty line after `.. code-block::` resulted in incorrectly
parsed restructuredtext (sphinx warnings) and the code blocks were not
generated in the html output.

The `exclude_patterns` change just silences the orphaned file warning.

[ci skip]

Decay types before passing to is_pod_struct<> / npy_format_descriptor<>

Report warnings as errors on Travis and AppVeyor (-Werror)

Add g++-6 builds to travis-ci

Fix format descriptors for string types

The current linux/g++ testing (using a backported g++-4.8 on a
4-year-old Ubuntu) is quite ancient.  It's good as a baseline level of
support, but it means we aren't testing g++'s C++14 support at all
(which is why #334 happened).

This commit adds a docker-based travis-ci build using the debian
"testing" distribution, which will give us both another test system
(with different versions of build tools), while, more importantly, also
adding a build and test run using g++ in C++14 mode.

This is required since format descriptors for string types that
were using PYBIND11_DESCR were causing problems on C++14 on Linux.

Although this is technically a breaking change, it shouldn't cause
problems since the only use of format strings is passing them to
buffer_info constructor which expects std::string.

Note: for non-structured types, the const char * value is still
accessible via ::value for compatibility purpose.

Add handle::repr() method

py::dtype, buffer protocol improvements, structured types support