refactor the Schedulable interface and add unit test for SchedulingAlgorithm

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

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

...la/spark/scheduler/cluster/FIFOTaskSetQueuesManager.scala

...la/spark/scheduler/cluster/FairTaskSetQueuesManager.scala

core/src/main/scala/spark/scheduler/cluster/Pool.scala

.../src/main/scala/spark/scheduler/cluster/Schedulable.scala

...in/scala/spark/scheduler/cluster/SchedulableBuilder.scala

...n/scala/spark/scheduler/cluster/SchedulingAlgorithm.scala

...c/main/scala/spark/scheduler/cluster/TaskSetManager.scala

core/src/test/resources/fairscheduler.xml

...rc/test/scala/spark/scheduler/ClusterSchedulerSuite.scala