Minor fixes to the documentation
JBPM-3913: Handle all exceptions in process instances with ExceptionScopeInstances if appropriate
JBPM-3913: Modified and added new WorkItemHandlers for exception handling

- also cleaned up invalid and unused code in jbpm-workitems
JBPM-3913: Extended support for event handling in event subprocesses

- event contents are now passed to the event sub-processes
- extended support for <error> and <signal> (and associated (error|signal)EventDefinition elements)
JBPM-3913: Exception handling tests (and clean up of other bpmn2 tests)

 - Tests for the SignallingTaskHandlerWrapper + Event Subprocess improvements
   - Tests for passing event contents to event subprocesses
 - Added the ExceptionService class that's used in the BPMN2-ExceptionServiceProcess* files
 - Moved some tests from JUnit 3 to 4 (@Test instead of "extends TestCase")
 - Remove duplicate TestWorkItemHandler implementations and refactored it to it's own class