Implement the Offer payload.
Use payloads.Offer wherever it's needed
Chat: minor doc improvement
Implement the Answer payload
Use payloads.Answer wherever it's needed
Implement the Hangup payload
Use payloads.Hangup wherever it's needed
Server: refactor the format of pushed events

An event is not an object with a topic and data attributes.
So {topic: "data"} becomes {topic: "topic", data: "data}.
This is more consistent with client-sides events

Moreover, the following "topics" changed:

- incoming_call becomes offer
- call_accepted becomes answer
- call_hangup becomes hangup
Merged with master
Implement the IceCandidate payload
Use payloads.IceCandidate wherever it's needed
SPA: fix Server#iceCandidate
Server: fix the iceCandidate API
Fix tests
Address pull requests comments
Minor test improvement
Fix a jshint error
Merge pull request #274 from tOkeshu/events-refactoring

Events refactoring