[SPARK-1341] [Streaming] Throttle BlockGenerator to limit rate of data consumption.

Author: Issac Buenrostro <buenrostro@ooyala.com>

Closes #945 from ibuenros/SPARK-1341-throttle and squashes the following commits:

5514916 [Issac Buenrostro] Formatting changes, added documentation for streaming throttling, stricter unit tests for throttling.
62f395f [Issac Buenrostro] Add comments and license to streaming RateLimiter.scala
7066438 [Issac Buenrostro] Moved throttle code to RateLimiter class, smoother pushing when throttling active
ccafe09 [Issac Buenrostro] Throttle BlockGenerator to limit rate of data consumption.