Ad skins - Front end and templates for admin

![screen shot 2014-02-19 at 10 28 59 am](https://f.cloud.github.com/assets/113293/2210020/d917d108-9993-11e3-9838-e63dde95d12e.png)

This completes the ad skins for Reddit.tv: Skins behave as normal advertisement campaigns with start and end dates, and can appear in the header, behind the video, and behind the channels list. Photoshop templates and instructions for making the skins are included in this commit, with quick download links in the admin section for ad skins.