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Add unit tests and documentation for the chrono cast.

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]

Use simple asserts and pytest's powerful introspection to make testing
simpler. This merges the old .py/.ref file pairs into simple .py files
where the expected values are right next to the code being tested.

This commit does not touch the C++ part of the code and replicates the
Python tests exactly like the old .ref-file-based approach.

Mark the pybind11 headers as private to the target.

Fixes #305

The function creates a pybind11 module using the specified source files.

This is needed in case an older pybind11 version was installed with pip

- automatically detect number of examples

- new pybind11::base<> attribute to indicate a subclass relationship
- unified infrastructure for parsing variadic arguments in class_ and cpp_function
- use 'handle' and 'object' more consistently everywhere

This modification taps into some newer C++14 features (if present) to
generate function signatures considerably more efficiently at compile
time rather than at run time.

With this change, pybind11 binaries are now *2.1 times* smaller compared
to the Boost.Python baseline in the benchmark. Compilation times get a
nice improvement as well.

Visual Studio 2015 unfortunately doesn't implement 'constexpr' well
enough yet to support this change and uses a runtime fallback.