Python3 support for oauth2client

Moving to a new PR rooted at the python3 branch in this repo to make commenting easier (as suggested by @methane). This PR will close #34 and #25.

Happy to accept new PRs against the python3 branch until this is merged. If you have something that needs work in both branches, feel free to do separate PRs for now.
This code is bad because `str(b'xyz') == "b'xyz'"` on Python 3.
@craigcitro Can I drop supporting PyOpenSSL<0.14?