BATCH-2009 - Stoppable tasklet
* Added the StoppableTasklet interface (an extension of the Tasklet interface).
* Updated JobOperator#stop(long executionId) to call StoppableTasklet#stop() on any currently running tasklets (local executions only).
* Updated the SystemCommandTasklet to implement StoppableTasklet
Merge pull request #173 from willschipp/BATCH-2009

* BATCH-2009:
  BATCH-2009 - Stoppable tasklet * Added the StoppableTasklet interface (an extension of the Tasklet interface). * Updated JobOperator#stop(long executionId) to call StoppableTasklet#stop() on any currently running tasklets (local executions only). * Updated the SystemCommandTasklet to implement StoppableTasklet