Homepage cover photo

Wanted to get this in in a separate pull request, because the circular project displays are going to take some intense svg stuff, probably. This PR implements the cover photo for the homepage (see images below). I'm still not totally sold on all the styles (they can definitely be improved, not sure how), but this is a good start, I think. We can talk about adding a link to a learn more page to the cover photo inset also, I think that would be a good thing. Note I darkened the images from the mockup, since I thought the text was a little bit hard to read over the fully lighted image.

Desktop: 
![screen shot 2014-12-02 at 12 57 50 pm](https://cloud.githubusercontent.com/assets/1168853/5270770/03dbc4d8-7a23-11e4-9565-283f24719008.png)

Tablet portrait:
![screen shot 2014-12-02 at 12 58 01 pm](https://cloud.githubusercontent.com/assets/1168853/5270773/10f8eef2-7a23-11e4-8139-88da87415c5b.png)

Phone landscape:
![screen shot 2014-12-02 at 12 58 08 pm](https://cloud.githubusercontent.com/assets/1168853/5270784/1f4b21a0-7a23-11e4-92f6-d3b9933ab3b9.png)

Phone portrait:
![screen shot 2014-12-02 at 12 58 17 pm](https://cloud.githubusercontent.com/assets/1168853/5270789/2762a0a2-7a23-11e4-9003-d8c926b80903.png)