Ability to have placeholder content in ad slot

Previously, this was happening (placeholder content wasn't being removed)

![screen shot 2014-09-15 at 15 05 12](https://cloud.githubusercontent.com/assets/74132/4272336/66019c7a-3ce1-11e4-967a-1abe6fae55d1.png)

Now it's beautiful

![screen shot 2014-09-15 at 15 04 53](https://cloud.githubusercontent.com/assets/74132/4272338/6af52684-3ce1-11e4-95b2-6f926cc88914.png)