Default getUserMedia maxFramerate set to 15fps

Configurable via options.maxFramerate.

It woulde be also interesting to use the googCpuOveruseDetection
flag (https://code.google.com/p/webrtc/issues/detail?id=2677,
https://code.google.com/p/webrtc/issues/detail?id=1577)

Change-Id: Ib5bd7c43f3bde2e1362a7eba00632f37845f1082