Ignore all .DS_Store files, not just the one in the project root.

This has been annoying me for a long time. The gitignore file currently includes `/.DS_Store`, but that only ignores the `.DS_Store` file in the project root. By removing the forward slash, all `.DS_Store` files can be ignored, so that people on OS X don't get a whole load of untracked `.DS_Store` files clogging up their `git status`, stopping them from using `git add .` or `git commit -a`.