Revise requestAnimationFrame scheduling.

This avoids queueing extra rAF callbacks if the queue bounces between
0 and 1 length. Also removed unnecessary 'webkit' prefix.
Use channel_model block instead of noise_source in SimulatedSource.

I don't know that it actually makes any difference for this purpose,
but if nothing else it's closer to expressing the intent of the
SimulatedSource.
Document Twisted and txWS deps.
Remove bogus str() which prevented rootCap = None from working.