Update CMake to allow for easier manual configuration of Python.

Users can change all variables as they see fit if the PYGMO_PYTHON_FINDER is turned OFF.

Behaviour:
_FINDER is ON by default.
_FINDER turns OFF when CMake can't find the *exact* version requested by user (this includes, major, minor and revision), but will suggest the best match. Example: user requests 2.7, CMake finds 2.7.6, reports mismatch and turns off finder.
_FINDER can be turned off manually. 

If the finder is off PYTHON_EXECUTABLE, PYTHON_INCLUDE_DIR, PYTHON_LIBRARY should be set manually.