Download images in the background.

Fixes #8

The GUI was unresponsive while downloading, so I put the actual image
retrieval into a thread.

For some reason it wasn't working but then for some reason it started
working.  I ahve no idea, all I changed was a bit of the debugging print
statements.  I have to assume that it was something to do with the way
printing (and logging) are handled in the presence of threads and the TCL
interpreter which is embedded and running the GUI.