Index: Better-looking dropdown icon.

Before:
![screen shot 2014-12-15 at 10 57 19 am](https://cloud.githubusercontent.com/assets/81969/5438843/2c3673d0-8449-11e4-8740-5680adfa2bbd.png)

After:
![screen shot 2014-12-15 at 10 57 11 am](https://cloud.githubusercontent.com/assets/81969/5438842/2c309f00-8449-11e4-8bf7-68e3bbb3a233.png)