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)

Also wondering if it would make sense to move the generation of the 'category' portion of the querystring to the activity_snippet macro. Or could combining it with that macro's querystring param be problematic?