WIP: Minimal implementation of stream groups for variant playlists

* Add a `group` option on streams, which is used on the slave side to create
  Stream Groups.  These are then used to generate variant playlists for
  HTTP Live Streaming
* Add `bandwidth` and `codec` options on streams to use in HTTP Live Streaming
  variant playlists
Clean up how we get stream variant options
Add admin support for new stream attributes

* Add `group`, `bandwidth` and `codec`
Merge pull request #23 from StreamMachine/StreamGroups

Pull in Stream Groups