Cffi cleanups

This bumps the cffi dependency to 0.5, makes the necessary changes to work with that version (primarily changes to how to extra byte strings from cffi buffer objects), and removes some usage of cffi private APIs (in tls/c/__init__.py).

The test suite is passing except for 4 tests which claim "mock.call" does not exist - I think because I have a too-old version of python-mock.