Coalescer now uses current preferred locations for derived RDDs. Made run() in DAGScheduler thread safe and added a method to be able to ask it for preferred locations. Added a similar method that wraps the former inside SparkContext.
comment in the test to make it more understandable
Merge pull request #1 from alig/coalesce-dagscheduler-locality

Coalesce now uses DAGScheduler to find out *current* preferred locations, i.e. support locality for derived sets