Improve Home page by adding a section on what FormatJS is

This adds a new section right under the splash section of the Home page that describes and visually shows _what_ FormatJS even _is_. This also adds a call-to-action "Get Started" at the end of the Home page content.

![screenshot 2014-10-03 12 18 26](https://cloud.githubusercontent.com/assets/29096/4508797/effc1f28-4b18-11e4-9d6f-28afe9410f2a.png)

![screenshot 2014-10-03 12 18 54](https://cloud.githubusercontent.com/assets/29096/4508805/fc69715c-4b18-11e4-8346-cfe8532a6258.png)