- added JobType to JobGraph and ExecutionGraph
- added interface Stoppable, applied to SourceStreamTask
- added STOP signal logic to JobManager, TaskManager, ExecutionGraph
- extended Client to support stop
- extended Cli frontend, JobManager frontend
- updated documenation

Fix JobManagerTest.testStopSignal and testStopSignalFail

The StoppableInvokable could not be instantiated by Task because it was declared as a private
class. Adds additional checks to verify that the stop signal behaves correctly.

Auto-detect if job is stoppable

A job is stoppable iff all sources are stoppable

- Replace JobType by stoppable flag
- Add StoppableFunction and StoppableInvokable to support the optional stop operation
- added REST get/delete test (no extra YARN test -- think not required as get/delete is both tested)
- bug fix: job got canceld instead of stopped in web interface
- Add StoppingException
- Allow to stop jobs when they are not in state RUNNING

Second round of Till's comments