janus.js: "connectionState" callback added

[RSATom]

it can be useful since latest Chrome versions
doesn't assign failed state to ICE connection state.

[lminiero]

Just an editorial note: maybe pcState would be a shorter and "better" name? We already shorten PeerConnection to pc in other places (e.g., pcStuff), and the other state related callbacks have a short name too. Or maybe let's just call it connectionState, since that's what the actual callback is?

[RSATom]

I've thought about using connectionState but worried it can be confusing for users since they can think it's related to http/WebSockets connection. But if you think it's ok - I'll change it.

And I don't like too significant abbreviation (I'm about pcState) but anyway it's up to you what exactly to choose - I'm ok with anything allowing me to fix issue :) - just tell me what you prefer...

[RSATom]

@lminiero FYI peerConnectionState -> connectionState (and the same for #3342 )