Proposal for BusinessProcess: add methods flushTask() and stopTask() for more fine-grained task managament

We would like to have the ability to have a "Save" button in our task forms which persists current task status and variables to the database without completing the task.

Additionally (as we plan to use Camunda BPM within Liferay portal) we need to switch the current active task within a view with another one. Here we would need some kind of stopTask() which flushes the task like above and additionally disassociates the current task from the BusinessProcess so that we can directly afterwards start a(nother) task using startTask().

This pull request is a code proposal that of course needs to be reviewed.

Company: Dräxlmaier Group (we'll have a subscription soon - Bernd and Jakob know about)