added a spinner for sync pull requests button which has a transparent bg

Before:
![before](https://cloud.githubusercontent.com/assets/1220480/5303494/11b272ac-7c10-11e4-9121-22ce3cfb3c7f.png)

After:
![after](https://cloud.githubusercontent.com/assets/1220480/5303499/1a5c1098-7c10-11e4-9900-4e80b2322979.png)