Update DataChannel.js

onmessage/onopen/etc. listeners were impossible to override, the default stub in onopen had an invalid send() call
Merge branch 'master', remote-tracking branch 'upstream/master'
delete channels onclose to avoid issuing send() over them later
Merge pull request #24 from mvayngrib/master

"leaveChannels" function needs a little bit more improvements. See those changes here:
https://webrtc-experiment.appspot.com/DataChannel.js

"RTCDataChannels" array also needs to be truncated each time when a user leaves.