Add ability to send signals

[stephanpelikan]

As a user of VanillaBP SPI I want to use Signals to start workflows or to correlate workflows waiting for a Signal.

1. There is a method ProcessService<WA>#broadcastSignal(String signal) to send Signals.
2. There is a method ProcessService<WA>#broadcastSignal<AU super WA>(AU aggregateUpdate, String signal) throws UnsupportedOperationEcxeption to send Signals. Since the workflows waiting for a Signal are selected by the underlying process-engine it is up to the specific adapter implementation to set the aggregate updated values. If a specific adapter is not able to fulfill this an UnsupportedOperationEcxeption is thrown.
3. Although the ProcessService bean is bound to a certain workflow aggregate WA (and therefore to BPMNs the workflow is bound to via @WorkflowServiceannotation) the Signal is not limited to the associated BPMNs.
4. If a Signal causes a workflow to be started (Signal start event), the aggregate's ID has to be extracted from aggregateUpdate or has to be set based on a value generated by the process-engine (checkout Signal issues in each specific adapter implemenation for details).