support audio/video in content streams

refactored class names:
- `SBAudioUpload` -> `SBAudio`
- `SBVideoUpload` -> `SBVideo`

added JSON transformers for expanded properties on a couple of models. considering converting audio/video models to `SBContent` subclasses.
Merge branch 'master' of github.com:stagebloc/CocoaBloc
ignore app delegate for test app
like/unlike for content
proper parameter assertions on like/unlike
get users who like content for 3 supported types
get comments for content, comment model, ...

get comments will deserialize to sbcomment. also added a params dict arg to the methods so we can pass limit/offset etc
add new headers to umbrella header
Merge branch 'master' into camera