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 48 25](https://cloud.githubusercontent.com/assets/29096/4509216/2d2387c0-4b1d-11e4-826a-5caac121161c.png)
![screenshot 2014-10-03 12 48 36](https://cloud.githubusercontent.com/assets/29096/4509217/31f9884e-4b1d-11e4-951b-255f4c560055.png)
![screenshot 2014-10-03 12 48 47](https://cloud.githubusercontent.com/assets/29096/4509218/37c35232-4b1d-11e4-8941-e8c4b7d1a6a3.png)