Untrack temporary, backup, .out and MacOS directory files

Sometimes text editors and IDEs produce temporary or backup files during editing. I think these files should not be tracked by git therefore I added them to `.gitignore`.

`.DS_Store` files are used by OS X to store custom attributes of a folder such as the position of icons or the choice of a background image ( http://en.wikipedia.org/wiki/.DS_Store )

`.out` files are sometimes produced as C executables or shared libraries ( http://en.wikipedia.org/wiki/A.out )