Deal with the encoded/unencoded distinction in MediaStream

[Manishearth]

Currently, GStreamerMediaStream can contain either encoded or raw audio/video. The current situation is:

• streams produced by webrtc are raw
• streams consumed by webrtc must be encoded
• media capture starts out raw but we produce it encoded so that it can be fed to webrtc
• player consumes streams encoded

IMO we should make it so that all streams are raw by default, but you can choose to stick in an encoder pipeline if you wish.