Implemented optional instrumentation for AsyncTask class
Introduced "TaskStateChangedEventHandler" into "DynamoScheduler"
Integrate task state change notifications to various scheduler parts
Integrated notification for cases when an AsyncTask is discarded
Tweaked exception handling around AsyncTask.HandleTaskCompletion
Handled "DynamoScheduler.TaskStateChanged" event in "DynamoModel"
Removed code around "AsyncTask.enableInstrumentation"
Moved "TestPreAndPostExec" test case out of "Failure" category
Added "TestTaskStateChangedEventHandling" test case for task events
Added documentation for "DynamoScheduler.TaskStateChanged" event
Merge pull request #2834 from Benglin/Revit2015

Cross merging master changes into Revit 2015