image_window in Python.

normalize_image_gradients(), line, signed_distance_to_line(),
distance_to_line(), reverse(), intersect(), count_points_on_side_of_line(),
count_points_between_lines(), dot(), and normalize() to Python API.

automatic binding.

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 is useful for visualizing the tracking results.

Also just cleaned up a few minor details.

A little bit hacky, but should be fine. Supports both fhog
detectors and the "cached" simple_object_detector. Also, maintains
the upscale parameter for testing

I also cleaned up a bunch of code. I'm not sure why the
simple_object_detector was keeping track of the upsample amount,
since it can't even be passed as an argument to the constructor.
Therefore, I removed the simple_object_detector_py and the second
declaration of the hog object detector. I also changed the
view code to optionally take keyword args of color and added
a single view of a rectangle.

Finally, I added viewing of the shape parts.