Initial version of an advanced Keen Callback that includes detailed
event information to allow better callback processing.

This implementation assumes that the both callback methods should be
called for each event (if the callback is an instance of the
DetailedCallback interface). This implementation calls the KeenCallback
callback method first, then the KeenDetailedCallback callback method
afterward.

Closes issue #32.
Merge pull request #36 from mikereinhold/feature-callback

Extension of KeenCallback interface to support callback execution with detailed information