Stream (Twitch) integration

Setup:
1. Add TWITCH_CLIENT_ID to config.py (can be left as empty string, but we might be subject to rate limiting from Twitch then)
2. As always manage.py syncdb
3. Add Twitch streaming platform in the admin panel with name "Twitch". Only Twitch is supported right now.

Special streaming pages can be reached under /streams/ (list of streams) and /streams/add/ (add, edit and delete streams). The homepage should list the 7 online streams with the most viewers.