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).
Changing line 169 to:

`"The Bag for folder %s has been created." % folder_picker_dialog.get_filename()`

Appears to have eliminated the error. I didn't get the tag error you reported. Did you add some custom tags?