Updated cv2.cpp to support Python 3 API.
Added a header with defines to enable the module to compile with either
python 2 or 3 without changes.
Updated cmake files to use print().
In these cases, just using print() is enough to ensure py2/3
compatibility.
Updated usage of METH_X definitions for python 3.
Python 3 requires METH_VARARGS | METH_KEYWORDS, not just METH_KEYWORDS.