Pluggable task assignment

jBPM should provide pluggable assignment strategies for user tasks to allow to cover various assignment strategies to be implemented both out of the box and by users.

Assignment of tasks means that based on some criteria task should be directly assigned to single user - in other words to have assigned actual owner so there won't be any need to claim manually by the potential owners.

At the same time assignment should be invoked when task is returned back to the "queue" like when released by current actual owner or reassigned or delegated.