Use OAuthlibs is_secure_transport method instead of duplicating it.

The method was only recently added to oauthlib.oauth2 (0.6.2) so
a version bump in requirements/setup.py is needed.
Stop piggybacking on oauthlib logging. Fix #85.
Add debug logging to oauth 1 session and auth.
Raise more informative errors when provider returns non urlencoded error responses. CC #124.
Add extra target for setup.py for RSA rather than defaulting to it. CC #120.
Merge branch 'master' of https://github.com/requests/requests-oauthlib
Use newly exposed oauth2.is_secure_transport instead of duplicate.