Refresh homepage

Closes mysociety/sayit#308.

I'm not overwhelmingly happy about how the screenshots are handled, but at least it gives some insight into how the product works, and we can always improve them later.

![sayit-bringing transcripts into the internet age 20140725](https://cloud.githubusercontent.com/assets/739624/3704351/f4e06aae-1412-11e4-8e5d-7b7e79336137.png)