Changed tab size to 4 spaces.

My .vimrc now specifies that softtabs, hardtabs and indents should all
be 4 spaces long. This is combined with the existing options to ensure
that spaces are used, instead of tab characters.
Added alias to clean Finder's "Open With…" menu.

Finder's "Open With…" menu often becomes cluttered with duplicates. The
command that I have now aliased to `owclean` will remove all of these
duplicates (and restart Finder for you).