1. Renamed PYTHON_* to PYBIND_* everywhere

2. Renamed pybind::array_dtype<> to pybind::array_t<>

simplified string casters, removed wide char support (inconsistent across platforms, better to use utf8 in any case..)

- Get a descriptive string after a Python exception (without changing the exception status)
- Convenience function to map from a C++ object to a Python handle
- Convenience to check if a pybind::function is defined by an underlying
  C++ implementation
- Get the type object of a pybind::handle