[webrtc-stats] Pull Request: Added more datachannel counters, with definitions.

alvestrand has just submitted a new pull request for 
https://github.com/w3c/webrtc-stats:

== Added more datachannel counters, with definitions. ==
This splits the datachannel open events into outgoing and incoming,
and the close events into "I close" and "others close".

Closes #79

See https://github.com/w3c/webrtc-stats/pull/125