Added subclass of UIModel which requires confirmation that the view has
been updated before allowing further image transitions via next_obs
or previous_obs.
Require acknowledgement before allowing transition to next source or
previous source.
Added ability to freeze a StatefulCollection, causing requests for 
transitions between items to be ignored.
Added freeze and unfreeze methods to WorkUnits.  
TransitionAcknowledgementUIModel can now simply freeze its current 
WorkUnit while waiting for an acknowledgement.
Ignore accept/reject current item requests while waiting for 
acknowledgement of image transition.
Don't explicitly keep track of the reading that the model is waiting to be
displayed.  Due to the nature of the freezing there should be no mix ups.
Controller now acknowledges the model when it displays an image.
Switched app over to using the transition acknowledging model.
Renamed TransitionAcknowledgementUIModel to TransAckUIModel.
Make sure no more CHANGE_IMAGE events can be sent while the model
is waiting for acknowledgement.
Reworked mechanism for ignoring accept/reject during transitions so that 
when processing candidates accept/reject is not ignored during an 
observation transition (only a source transition).  Otherwise the behaviour 
is the same, with further transitions being ignored, and accept/reject
being ignored for during any transition while processing reals.
Made CandidatesWorkUnit create its writer lazilly.  Cleaned up some test 
cases that had cross-interactions.
Removed freeze feature from WorkUnit and StatefulCollection because 
re-implementation of the TransAckUIModel doesn't use it.
Merge pull request #160 from drusk/transition_acknowledgement

Closes #138.