Overriding field names when binding structured dtypes

PYBIND11_NUMPY_DTYPE_EX(Type, F1, "N1", F2, "N2", ...)

* Make reference(_internal) the default return value policy for properties

Before this, all `def_property*` functions used `automatic` as their
default return value policy. This commit makes it so that:

 * Non-static properties use `reference_interal` by default, thus
   matching `def_readonly` and `def_readwrite`.

 * Static properties use `reference` by default, thus matching
   `def_readonly_static` and `def_readwrite_static`.

In case `cpp_function` is passed to any `def_property*`, its policy will
be used instead of any defaults. User-defined arguments in `extras`
still have top priority and will override both the default policies and
the ones from `cpp_function`.

Resolves #436.

* Almost always use return_value_policy::move for rvalues

For functions which return rvalues or rvalue references, the only viable
return value policies are `copy` and `move`. `reference(_internal)` and
`take_ownership` would take the address of a temporary which is always
an error.

This commit prevents possible user errors by overriding the bad rvalue
policies with `move`. Besides `move`, only `copy` is allowed, and only
if it's explicitly selected by the user.

This is also a necessary safety feature to support the new default
return value policies for properties: `reference(_internal)`.

The current integer caster was unnecessarily strict and rejected
various kinds of NumPy integer types when calling C++ functions
expecting normal integers. This relaxes the current behavior.

Prevent overwriting previous declarations

Currently pybind11 doesn't check when you define a new object (e.g. a
class, function, or exception) that overwrites an existing one.  If the
thing being overwritten is a class, this leads to a segfault (because
pybind still thinks the type is defined, even though Python no longer
has the type).  In other cases this is harmless (e.g. replacing a
function with an exception), but even in that case it's most likely a
bug.

This code doesn't prevent you from actively doing something harmful,
like deliberately overwriting a previous definition, but detects
overwriting with a run-time error if it occurs in the standard
class/function/exception/def registration interfaces.

All of the additions are in non-template code; the result is actually a
tiny decrease in .so size compared to master without the new test code
(977304 to 977272 bytes), and about 4K higher with the new tests.

NumPy scalars to ctypes conversion support

This reduces direct access to internals.registered_types_cpp to
just a few places.

This avoid a hashmap lookup since the pointer to the list of
direct converters is now cached in the typeinfo.

Bugfix: bad delete if no copy ctor

* Remove obsolete example reference
* Make example fully-working (except for #includes)

Fixes #456.

Fix def_property and related functions

Making `cppfunction` explicit broke `def_property` and friends.
The added tests would not compile without an implicit `cppfunction`.

Without the previous commit, this test generates a core dump.

type_caster_generic::cast(): The values of

    wrapper->value
    wrapper->owned

are incorrect in the case that a return value policy of 'copy' is
requested but there is no copy-constructor.  (Similarly 'move'.)  In
particular, if the source object is a static instance, the destructor of
the 'object' 'inst' leads to class_::dealloc() which incorrectly
attempts to 'delete' the static instance.

This commit re-arranges the code to be clearer as to what the values of
'value' and 'owned' should be in the various cases.  Behaviour is
different to previous code only in two situations:

policy = copy but no copy-ctor: Old code leaves 'value = src, owned =
true', which leads to trouble.  New code leaves 'value = nullptr, owned
= false', which is correct.

policy = move but no move- or copy-ctor: old code leaves 'value = src,
owned = true', which leads to trouble.  New code leaves 'value =
nullptr, owned = false', which is correct.

Support std::shared_ptr holder type out of the box

Reorganize documentation

With this there is no more need for manual user declarations like
`PYBIND11_DECLARE_HOLDER_TYPE(T, std::shared_ptr<T>)`. Existing ones
will still compile without error -- they will just be ignored silently.

Resolves #446.

Auto-implement format/numpy descriptors for enum types

Make operator bool() explicit