stop polling if peerconnection is deleted, closed or ice failed

when connectivity fails, this currently keeps polling forever. 

Canary now goes to failed when TURN/TCP is blocked (https://code.google.com/p/webrtc/issues/detail?id=3249), at least on the initiator (see https://code.google.com/p/webrtc/issues/detail?id=1414).
uhm... apparently firefox increases the video's currentTime and therefore triggers the transition to active even before the ice connection is up?!

(apart from that, onaddstream sometimes fires with delay in Firefox which is a known bug and affects your TODO in https://github.com/GoogleChrome/webrtc/blob/master/samples/web/content/apprtc/js/main.js#L280)

I do have a patch ready for adapter.js though :-)