Changed naming of StageCompleted event to be consistent

The rest of the SparkListener events are named with "SparkListener"
as the prefix of the name; this commit renames the StageCompleted
event to SparkListenerStageCompleted for consistency.
Updated code style according to Patrick's comments
Merge pull request #308 from kayousterhout/stage_naming

Changed naming of StageCompleted event to be consistent

The rest of the SparkListener events are named with "SparkListener"
as the prefix of the name; this commit renames the StageCompleted
event to SparkListenerStageCompleted for consistency.