Added a .gitignore file.

This file allows git to ignore certain patterns, so that they don't appear in your calls to 'git status' and don't get commited into the history when you do 'git commit -a'. For now, I have added the matlab backup files. Feel free to add more patterns on separate lines (see here for details: http://www.kernel.org/pub/software/scm/git/docs/gitignore.html).