Gallery ads

Add adverts to the gallery page (after every 4th image)

![screen shot 2014-08-29 at 10 13 44](https://cloud.githubusercontent.com/assets/74132/4088418/df953052-2f5c-11e4-9855-ee0c85984b74.png)

What does this 300px correlate to? mpu dimensions size? maybe move to var to be more verbose.