Threaded replies in the feed view

I tried this to compare with https://github.com/pfraze/phoenix/pull/133 and I like both.... but I think I like this better.

This gives some really useful structure to the feed. What it loses is the ability to always see the most recent messages -- but I think we can make the inbox work for us there.

![screenshot from 2014-11-03 15 34 49](https://cloud.githubusercontent.com/assets/1270099/4890721/7a0e5e56-63a1-11e4-95a2-ae34f25087fd.png)