Adds experimental OSX support

A bit sketchy, but seems to mostly work.

Depends on PyObjCTools, which will be installed inside a virtualenv the first time ulogme.sh is run on OSX.

The biggest gotcha is that keypress events are not captured from Terminal.app. I don't have a very deep understanding of the OSX event model, so I'm not really sure what is going wrong. A dirty workaround is to use iTerm2 instead of Terminal.app if you care about logging shell keystrokes.