Add a gradient to buttons

Thoughts?

Before:
![image](https://cloud.githubusercontent.com/assets/253202/2868966/6ca89c88-d254-11e3-9cd7-fa3aeba018f8.png)

After:
![image](https://cloud.githubusercontent.com/assets/253202/2868964/5be8e29a-d254-11e3-8a7f-ca88c41fa79a.png)