Bug 1025872 — Setup websockets for call setup information

The API has been defined by @adamroach in this document: https://wiki.mozilla.org/Loop/Architecture/MVP#WebSockets_Connection_for_Call_Progress
Say we filter the messages here because we're receiving all the messages.

That actually worries me a bit that we're receiving all the messages, it means we will trigger cpu operations when we don't need it ...