Added a couple more test cases for cutout calculations.
Matplotlib image viewer now shows full image; previously some pixels 
were cut off at the top and bottom.
Tweaked image viewer bounds to give a small border region.
Added cands.astrom file which showed cutout problem to 
tests/data/astromdir.
Made header for KeyValueListPanel parameterizable.
Added show keymap menu item which lists the keyboard shortcuts in 
a small popup window.
Gracefully handle the case where there is no work to be done at startup.
Writes a brief message to stdout explaining there is nothing left to 
process in the specified directory.
Added ability for MPC writer to write lines when photometry has failed
where the observed magnitude and band are left blank.
Photometry failure error handling.
If daophot cannot compute the magnitude when phot_mag is called
a TaskError is raised.  The controller catches this and launches the 
accept source dialog with the observed magnitude and band left blank,
and the comment pre-filled with the TaskError error message.
Merge pull request #94 from drusk/cutout_edges

Closes #74, #69.
Merge pull request #95 from drusk/keybind_list

Closes #92.
Merge pull request #96 from drusk/empty_workload

Closes #91, #42.
Merge pull request #97 from drusk/phot_failure

Closes #78.