Fix carousel control margin asymmetry

While using carousel default controls with a non-default background, I've noticed annoying asymmetry of arrows. Looks like somebody forgot to specify different margins for left and right arrows.

Before:
![2014-03-31 12 24 47](https://cloud.githubusercontent.com/assets/1201569/2565186/3dd18bbc-b8b0-11e3-85e9-df6d766e67d7.png)

After:
![2014-03-31 12 35 32](https://cloud.githubusercontent.com/assets/1201569/2565187/3f59203a-b8b0-11e3-92a3-6b5ef814d83e.png)