Add QOE tracking

[theodab]

trafficstars

We should track and expose QOE (quality of experience) information to the app. We should expose the following information:

• player.stats()
• How long until frames are rendered (from before load() to a segment downloaded, demuxed, decoded, and rendered)
• Real framerate
• Number/frequency of dropped frames (already tracked but not exposed)
• Frequency and duration of buffering stalls
• Buffer fullness
• Smoothness? (measured using buffering stalls, dropped frames, and how often we are delayed for decoding)