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).
Running it under Ubuntu results in the following error when I create the Bag:

`Traceback (most recent call last):
  File "createbag.py", line 169, in on_folder_clicked
    "The Bag for folder %s has been created." % bag_dir)
NameError: global name 'bag_dir' is not defined
`