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).
There is a Cocoa popup on success, but shouldn't there be one if there is an error creating the Bag (either from an error sent by the Bagit library, or more likely, a file permission error on the output directory)?