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).
Sorry, I didn't see that the path needs to be hard coded because of https://github.com/mjordan/createbag/pull/1/files#diff-04c6e90faac2675aa89e2176d2eec7d8R71. But is it essential that the application gets installed in the user's home directory? Is there a way to share one installation among many Macs?