Support statically defined callback functions

[javrasya]

Callback functions can today be registered dynamically by persisting the body of the method in the DB to conform even more to what django design idea is. But not all the workflow users are fond of this method where they can simply define callback methods statically from their workspace. It is good to have this available in django-river as well.

Acceptance Criteria:

• Offer new APIs to register callback methods statically.
• Document how these APIs can be used
• Also document how dynamic callbacks can be registered