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).
I think we should plan for bundling on all platforms. Can you figure out if that's feasible on OS X and if so, we can reopen this issue. For now let's go with the hard-coded installation directory and make sure it's documented.