Add editorial brand icons (bug 1031179)

I didn't like using an `<img>` for these, as was stubbed out, so I took a slightly different approach.

Some problems with what is here right now:

- Not all of the icons are not visually centered with `background-position: center center`. Probably best to merge this, then ask UX for specific `x y` values for each icon they aren't happy with.
- None of the icons came with color instructions, and there was never a color choice specced for editorial brands. They currently all have #0096DC.
- I took a good guess on the mappings from brand <-> icon, but it wasn't obvious, and some may not be as intended.
- This isn't in fireplace yet. Will copy these files once they're merged.

![screen shot 2014-07-08 at 10 18 45 am](https://cloud.githubusercontent.com/assets/23885/3511928/0ba7cc58-06b4-11e4-8bb4-dcd77b2310f3.png)
![screen shot 2014-07-08 at 10 18 39 am](https://cloud.githubusercontent.com/assets/23885/3511929/0babf788-06b4-11e4-8934-d74ff4f49d1b.png)

r? @ngokevin @spasovski @pwalm
I think we could get away with removing the top-level .feed-brand selector to remove some specificity