Improve Firefox compatibility

There are still many issues with Firefox, but this fixes some of them.
Testing with current stable versions of Chrome (27) and Firefox (22)
Believed to be working:
- Firefox to Firefox
- Chrome to Firefox
Not working:
- Firefox to Chrome (ICE candidates seem to be rather lacking)
- Downgrading video to audio on answer (Firefox doesn't seem to like the
  SDP)
- Firefox does not work with TURN (doesn't support credentials)
Merge branch 'master' of https://github.com/crocodilertc/crocodile-rtc