ported to OSX. not tested; almost definitely needs more work.
fixed some indenting
unbroke Gtk version, except for mysterious _make_tag_file regex type error in bagit lib
fixed success message
cleaned up function names.
added success message on OSX, and commented out cleaning of tmp directory since I didn't realize that's where bags will end up without being moved elsewhere
forgot to add termination after error dialog on OSX
removed commented-out tmpfile clearing for now
Merge pull request #1 from axfelix/master

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