Mini activities feed proof of concept

Throwing this up here prematurely so we can discuss the approach. It's pretty straightforward but just want to make sure that it makes sense and see if I'm missing anything. This also needs some styling, it's supposed to live in the grey footer area. I only added the Blog doc type and the Newsroom categories (as their own kind of pseudo doc types).

![image](https://cloud.githubusercontent.com/assets/1093373/5127198/a5f0cad4-70a0-11e4-928e-fd9724fe6223.png)

Updated in my new PR https://github.com/cfpb/cfgov-refresh/pull/209