Make the installer more friendly to OS X.
Prevent vim-pathogen errors on Mac OS X.

Vim on OS X complains if you do `filetype off` without having previously
done `filetype on`. The `filetype off` is required for Pathogen to load
properly, so we'll do these in pairs, even though it seems pointless.
Configure vim to use pathogen as a submodule.
Add some comments to the install script.
Ignore swap files.