Add clearfix class to gallery

... so the gallery will be cleared after the floated boxes.

before:
![before](https://f.cloud.github.com/assets/157533/269736/249ee57a-8faa-11e2-8175-6a2d1a9d9ace.png)

after:
![after](https://f.cloud.github.com/assets/157533/269737/24b6fa48-8faa-11e2-941b-400297af1c42.png)