Renamed StandaloneX to CoarseGrainedX.

(as suggested by @rxin here https://github.com/apache/incubator-spark/pull/14)

The previous names were confusing because the components weren't just
used in Standalone mode.  The scheduler used for Standalone
mode is called SparkDeploySchedulerBackend, so referring to the base class
as StandaloneSchedulerBackend was misleading.