capi: SONAME of libmockspotify = API major version
capi: expose the error code in SpError
capi: various fixes in session handling
Add a spotify.mock module to interface libmockspotify

The spotify.mock module is structured the same as spotify. There is an
internal API using ctypes to interface libmockspotify, and a higher level
Pythonic API that eases the creation of mock Spotify structures.

ex: mocksp_session_{create,destroy}(...) vs mock.Session()
tests: complete capi tests for session handling