A frame has to have Show() called on it before entering the wx app's
mainloop.  This was happening in debug mode (because of the 
inspection widget being shown) but not with debug off.
After downloading an image, if we find that the user has closed the 
application, don't try to run the callback because the rest of the
application except for this download thread has been shut down.  
This avoids PyNoAppErrors and PyDeadObjectErrors which would make
the user uncomfortable.
Merge pull request #19 from drusk/issue17

Issue17