Python3 compatibility

[evilaliv3]

While working on GlobaLeaks and evaluating twistar usage (globaleaks/globaleaks-whistleblowing-software#1551) we are now evaluating the code quality of twistar in relation to python3.

This ticket is to keep track of the activities related to make twistar compatible with python3.

@bmuller: With commit #59 i've now made possible to test against python3 and it should be easy now for you to fix the support by simply looking at the python3 errors at: https://travis-ci.org/evilaliv3/twistar

\cc @fpietrosanti