@zamiang: Polish up text section editing. More state things

For context I'm using [React](http://facebook.github.io/react/) to do the section UI of the new posting tool. In [this PR](https://github.com/artsy/positron/pull/25) I explain the findings and decision to move forward with it.

I stumbled a bit when I tried to make it so that when you add a new section it should focus inside the last added section (SectionTool component communicating to the TextSection component). After scouring docs some more it seems like React tends to recommend when in doubt move state up the chain. So I refactored the "editing mode" for sections to be handled by the top level view.

This brings lots of other visual and state polishes to get adding/removing text sections closer to feeling good.

Also:

* Refactored to use a real Backbone collection
* Move the section + article merging into the model
* The internal API was saving updated_at as moment.js objects :-P so needed to `.format()` that
* 

![cap](https://cloud.githubusercontent.com/assets/555859/4683238/d7e299c8-5624-11e4-9e84-401326de7d5a.gif)