prepare for python3 compatibility

Tito is not listed as a blocker for "python3 by default"
at https://fedoraproject.org/wiki/Changes/Python_3_as_Default
but I'm rather fond of tito and wanted to give a shot at
making it python3-compatible.

The end result is a bunch of pep8 tests to clean up the code
and get as close to python3-compat as I know to do
without a python3-git library.
    
This changeset does *not* assert python3 support since
GitPython is not compatible with python3 and may never be:
https://fedoraproject.org/wiki/User:Churchyard/python3

However, it *does* work on rhel5, rhel6, fedora19, and fedora20.

View diff on github without whitespace changes at
https://github.com/dgoodwin/tito/pull/105/files?w=1

Feedback?