Remove .DS_Store files

Remove .DS_Store files. For those of us on a Mac, I recommend the
following:

Create a ~/.gitignore_global file containing:
.DS_Store

Execute the following command:
git config --global core.excludesfile ~/.gitignore_global

This assumes that you don't already have an excludesfile specified
in ~/.gitconfig

Signed-off-by: Jeff Godin <jgodin@tadl.org>