Add arrow to header expand/contract

![image](https://cloud.githubusercontent.com/assets/206973/5369872/287e092a-7fcf-11e4-9576-65a5f803d58e.png)

![image](https://cloud.githubusercontent.com/assets/206973/5369877/30c9c718-7fcf-11e4-8eac-09293e747cbe.png)