Added basic dialog for retrying downloads.
added a force variable to make starting all over easier"
mistakenly commited version with forced exit for testing
corrected syntax error in print statement
made logging message in library debug level
Merge branch 'master' of https://github.com/ijiraq/MOP
corrected reference to option flag
added more infor to logging message to clarify origin or message"
ensure magfile is deleted before using
added force option
add force as option
check that enough values passed before setting force
Added mechanisms for showing the retry download dialog.  Performing
the retry is still to be implemented.
Created DownloadableItem class to better abstract the download context
for each image.  This should make it easier to implement download 
retries.
It also allows more of the logic related to determining the cutout to be 
decoupled from the downloader.
Added get_image_uri and get_apcor_uri methods directly to the 
Observation class since the calculation is based on its fields.  
SourceReading also has these methods now, delegating to its Observation.
Removed usage of VOSpaceResolver class.
Reworked thread managment for downloads.  The same worker threads
are now re-used, pulling from a work queue.
Renamed VOSpaceErrorHandler to DownloadErrorHandler.
Retry-download feature now working end-to-end.  However, if one's 
network connection goes down it creates a flood of popups which should
be remedied.
Don't use daemonized threads because weird error messages get 
dumped if you shut down the application while mid-download.