previous dlib versions.

conform to dlib's generic image interface.  This makes dealing with
numpy images in pybind11 modules much nicer.

- Moved all uses of PYBIND11_MAKE_OPAQUE to a common header to avoid possilbe ODR violations.
- Added python binding for make_bounding_box_regression_training_data().

* Replace boost::python with pybind11

* Replace add_python_module with pybind11_add_module

* Fix clang error on type-dependent expression

This deals with converting python objects to dlib objects