Further refactoring, and start of a standalone scheduler backend

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

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

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

.../src/main/scala/spark/executor/MesosExecutorBackend.scala

...main/scala/spark/scheduler/cluster/ClusterScheduler.scala

...main/scala/spark/scheduler/cluster/SchedulerBackend.scala

.../main/scala/spark/scheduler/cluster/TaskDescription.scala

...n/scala/spark/scheduler/mesos/MesosSchedulerBackend.scala

...spark/scheduler/standalone/StandaloneClusterMessage.scala

...ark/scheduler/standalone/StandaloneSchedulerBackend.scala

core/src/main/scala/spark/util/SerializableByteBuffer.scala

spark-executor