highlight active dropdown with a checkmark

Fixes #664 

![screen shot 2014-08-11 at 1 29 08 pm](https://cloud.githubusercontent.com/assets/1656824/3882796/bbab3a6c-219a-11e4-8188-f3ff6218c239.png)
![screen shot 2014-08-11 at 1 29 11 pm](https://cloud.githubusercontent.com/assets/1656824/3882797/bdc898b2-219a-11e4-94e4-1251dc0d0753.png)
![screen shot 2014-08-11 at 1 29 14 pm](https://cloud.githubusercontent.com/assets/1656824/3882800/c1668b14-219a-11e4-90ea-70e149fdae81.png)