Add install button to readme

Uses my new project [do-install-button](https://github.com/seven1m/do-install-button). :smiley_cat: 

![screen_shot_2014-10-27_at_9_30_56_pm](https://cloud.githubusercontent.com/assets/669/4803045/124d76cc-5e50-11e4-9ee6-1f5c0306b5db.png)

![screen shot 2014-10-27 at 10 11 52 pm](https://cloud.githubusercontent.com/assets/669/4803048/1f9d4f64-5e50-11e4-8643-aea6447b620a.png)