Add OnPurge and OnDeleteStream to IPipelineHook. Add PipelineHookBase that provided optional default implementation of IPipelineHook.
Authorization, EventUpconverter and DispatchScheduler pipeline hooks inherit from PipelineHookBase, simplifies things a bit.
OptimisticPipeline hook implements OnPurge and OnDeleteStream to remove tracked commits from it's cache.
OptimisticPipelineHookTests - Fix test file name to match class name
PipelineHooksAwarePersistanceDecorator calls OnPurge and OnDeleteStream for all pipeline hooks + tests
Merge pull request #333 from damianh/Issue-332

Fixes Issue #332