SEO: related keywords on tag pages

Adds related keywords to tag pages for SEO.

![screen shot 2014-07-22 at 15 24 17](https://cloud.githubusercontent.com/assets/1001642/3659205/fcce1040-11ab-11e4-8595-f2a3e2a34551.png)

![screen shot 2014-07-22 at 15 24 44](https://cloud.githubusercontent.com/assets/1001642/3659206/0079efe8-11ac-11e4-95b9-b3279628aef0.png)

![screen shot 2014-07-22 at 15 24 57](https://cloud.githubusercontent.com/assets/1001642/3659209/0837a932-11ac-11e4-851e-fdadd0177181.png)

My design skillz suck. Kat and Alex aren't available to look at it at the moment, though.
in two minds as to whether the `,` should live here, rather than adding it with css