ported to OSX. not tested; almost definitely needs more work.

hey Mark,

rearchitected this to make it (hopefully) work cross-platform using the Automator to add a context menu entry and Cocoadialog in place of Gtk. not yet tested, but if you want to ensure I haven't broken it on Linux/Windows, that wouldn't be a bad idea... (I'll check myself later on).
from "most recent commit "finishes" the OSX version (it creates a bag and provides a success message), and I think all other issues should be fixed at this point!

I commented out the cleaning of the /tmp directory I'd added earlier today; originally, I put that in there because I noticed while testing that if I kept trying to create the same bag, it would keep creating arbitrarily many /data directories in the same subfolder in /tmp. I think the script should probably finish by moving the /tmp bag to the desktop or the same directory as the original one so that /tmp can be cleaned -- what do you think?"

I left the commented out code in there so that I'd remember to revisit this, though everyone always tells me that's bad practice :)