Faster sniffing

Faster sniffing

- Sniffing now goes through the old extension-based filter before
  doing it the bitsniffing way.
- Refractored get_media_type_and_manager(filename).
- Removed ogg extension from video accepted extensions, audio
  will take care of that.
- Added custom audio player, still WIP,but working.
- Added test for sniffing. This only tests for the
  mediagoblin.media_types.image type, as that is the only
  one enabled from start.