Add installation for OSX applications

This pull request adds a `osx-install.sh` script which handles converting `nbmanager` into an OSX application, and then symlinks the application to the appropriate place.

This also adds `nbmanager.py` because I don't think it will work with `python3 -m nbmanager` -- it really seems to want an actual python file that it can execute. Unfortunate, though, because then you have pretty much the same thing twice :-/