Some refactoring to make cluster scheduler pluggable.

core/src/main/scala/spark/PairRDDFunctions.scala

core/src/main/scala/spark/SparkContext.scala

core/src/main/scala/spark/TaskContext.scala

core/src/main/scala/spark/TaskState.scala

core/src/main/scala/spark/Utils.scala

core/src/main/scala/spark/deploy/DeployMessage.scala

core/src/main/scala/spark/deploy/ExecutorState.scala

core/src/main/scala/spark/deploy/JobDescription.scala

core/src/main/scala/spark/deploy/client/TestClient.scala

core/src/main/scala/spark/deploy/client/TestExecutor.scala

core/src/main/scala/spark/deploy/master/JobState.scala

.../src/main/scala/spark/deploy/worker/ExecutorManager.scala

core/src/main/scala/spark/deploy/worker/Worker.scala

.../src/main/scala/spark/deploy/worker/WorkerArguments.scala

core/src/main/scala/spark/executor/Executor.scala

core/src/main/scala/spark/executor/ExecutorContext.scala

core/src/main/scala/spark/executor/MesosExecutorRunner.scala

core/src/main/scala/spark/scheduler/ResultTask.scala

core/src/main/scala/spark/scheduler/ShuffleMapTask.scala

core/src/main/scala/spark/scheduler/Task.scala