Merge pull request #84 from tOkeshu/chat-window-call

Chat window call
Cherry-picked Romain's webrtc model
Fix all but one jshint issues; the last of which appears to be a real bug
ChatApp: refactor the app to have a _onIncomingCall handler
ChatApp: rename _onCallStart to _onStartingCall
ChatApp: create a sdp offer when starting a call
WebRTCCall: refactoring
ChatApp: establish the webrtc call when receiving an answer
ChatApp: create a sdp answer when receiving a call
Minor cosmetic improvements
ChatApp: minor test refactoring
ChatApp: fix typo
ChatApp: listen for talkilla.call-establishment events