Add .gitignore to remove DS_Store

I noticed you had some DS Stores.

![ycevkxm](https://f.cloud.github.com/assets/1504626/1558639/36ef1b90-4f9f-11e3-97eb-ef39bced2779.gif)

Attached is a pull request to remove them and a .gitignore to make sure you don't do that again!