Use SPARK_MASTER_IP if it is set in start-slaves.sh.

Removing credentials line in build.

Adding dependency repos in quickstart example

Let the user specify environment variables to be passed to the Executors

Also removed unused variables in the ExecutorRunner.

- Added a StorageLevels class for easy access to StorageLevel constants
  in Java
- Added doc comments on Function classes in Java
- Updated Accumulator and HadoopWriter docs slightly

Change block manager to accept a ArrayBuffer 

memory

Adding Java documentation

that the computation can proceed even if we run out of memory to cache the
block. Update CacheTracker to use this new interface

Adding code for publishing to Sonatype.