Added WebRTCCall to help establishing a calling between two local peer connections.

Implemented a DataChannel bandwidth test that sends a block of packets of 1024 bytes
everytime the bufferedAmount is 0 and prints the bitrate sent over one second.
Added suport for the call to run via TURN and fixed comments.
Converted output to bits and added test description.
Merge remote-tracking branch 'origin/master' into datachannel-bandwidthtest
Merge pull request #117 from GoogleChrome/datachannel-bandwidthtest

DataChannel bandwidth test and WebRTCCall to help doing loopback calls.