@gib: Initial Artwork Sections

This adds an artwork section component that can be used to embed artworks in posts. Currently it uses stubbed data b/c I need to do some trickery with the API. For context Positron has it's own mongo backed API that can do some aggregation-layer stuff, and in this case I want to circumvent some issues with using v2 directly.

I also refactored the section's "hover controls" into a "section container" component that handles the editing states.

I'm stumbling a bit with testing React components so please excuse the pending tests. I'll get to fixing them up later.

![cap](https://cloud.githubusercontent.com/assets/555859/4746319/bed52348-5a47-11e4-897b-5150b8101a3e.gif)


React is really cool. However, it does definitely require a learning curve that I think is not necessary for most projects. You can see my rant here: https://github.com/artsy/positron/pull/25 about it. So far I think it removes a lot of boilerplate and state management—which is a big win, but on the other hand it requires a new mindset which can be cumbersome at times.