Small Improvements on HumanTask API

The currect org.jbpm.process.workitem.wsht package includes several different HumanTask implementations, most of them implements directly the org.drools.runtime.process.WorkItemHandler interface - although it works as expected the API can be improved by a set of small changes makes more clear the intent of each class.
Here are a couple of suggestions, in order to make the API more explicit for end-users:

1. Create a common HumanTask interface and all implementation inherits from it. This will allow users check easely (using any IDE) what are the available HumanTasks.
2. Create some marker interfaces (like Serialization) to give additional context for users of each task usage/scope/goal. Some examples of those marker interfaces would be PersistentWSHumanTask, RemoteWSHumanTaskHandler and LocalWSHumanTaskHandler.

Sure that both suggestions are already addressed somehow thru docs, but using a self describable API would be a big advantage.