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

* Supporting bottom-up DIBs

* Variable renamed

a part with an automatically selected integer label if and only if you haven't
specified any string part labels and all the existing parts are already integer
labeled.  This makes it way easier to quickly add parts.

that it's going to move the matrix into the result.

* Fixed reference count issue

* Fixed refcount issue in Python dlib.jitter_image and dlib.get_face_chips

* Consolidation of https://github.com/davisking/dlib/pull/1249

* Fixed build issue

* Fixed: Paths in a pytest file should be relative to dlib root

* Skip numpy return tests for Python 2.7 or if Numpy is not installed

* Enabled numpy returns tests on Python 2.7 using cPickle.dumps