Added documentation in workload.py.  Renamed 
StatefullCollection.add_callback to 
StatefullCollection.register_change_item_callback.
Made daophot module independent of GUI application data structures.
Removed unused exception class.
Created new cutouts module for the CutoutCalculator and 
CoordinateConverter classes.
Renamed imgaccess module to downloads.  Merged test cases for
downloads classes into one test module called test_downloads.
Renamed img module to image.  Renamed FitsImage class to 
DownloadedFitsImage.
Reworked events.  Got rid of navigation events, replaced with the 
CHANGE_IMAGE event because that is all we really care about at the 
moment.

Also moved firing of events into UIModel, which avoids the rapid jumping
effect that was occuring when the RealsWorkUnit did a fast-forward.
Refactoring: simplifying the image loading event process to use reading
objects instead of relying on source and observation numbers.