Provide API for accessing JBPM email infrastructure

Currently there is no way to access JBPM email infrastructure from custom Java code (for example from an EventListener, or ActivityBehaviour)

It would be good to be able to access JBPM email subsystem through a public API.

Adding a new Facade class with a single method that allows you to select the email template and pass an org.jbpm.api.Execution should be quite easy to implement and would add a lot of power to EventListeners, etc.