Merged functionality of WorkloadManager into AbstractModel.
Removed WorkloadManager class.  Changed a couple test cases for it to
test the model which now has that functionality.
Merged ProcessRealsModel and ProcessCandidatesModel into the 
superclass AbstractModel.
Renamed AbstractModel to UIModel.
Moved try/except block for NoAvailableWorkException to the actual
location where it occurs.  Cleaned up UIModel accept_current_item and 
reject_current_item methods.