Python3 compatibility

This PR adds python 3 compatibility to FlaskBB, as requested in issue #68.

It changes the following things:
- Replace Flask-and-Redis with Flask-Redis
- Use unidecode instead of ascii normalisation for slugs
- Fix other py3 related problems (iteritems, sys.maxint, unicode(), etc.)

I am not able to test the reworked redis functionality though (don't have any redis servers), so it would be wise to have someone testing that before merging with master.