Bug fixes and improvements from the use of this in the Talky iOS app.

Add support for audio only use.
Add support for setting STUN and TURN servers.
Broadcast a single local stream to all peers, and provide a public accessor.
Add state change method to signal delegate.
Add peer identifier to stream methods in signal delegate.
Make sure that callbacks from within WebRTC are all bridged to main thread.
Merge pull request #1 from steamclock/master

Bug fixes and improvements from the use of this in the Talky iOS app.