Homepage con mas

Home page design and copy changes worked on during Mozilla Open Science sprint, finally pull requested.  The goal was to create simplified, compelling marketing copy that made less reference to Github upfront - which not all scientists understand well or know about.  Also updates the "branding" with some blue and a stronger call to action by making the signup button green.

Thanks to @szeitlin and @dtrapezoid for their awesome input on the new copy and design changes!

Here's a preview:

![scientific_protocols_-_a_free_and_easy_way_to_share_scientific_protocols_and_scientific_protocols_-_a_free_and_easy_way_to_share_scientific_protocols](https://cloud.githubusercontent.com/assets/5185/4519352/943a3918-4cb7-11e4-852b-c8a7fece454b.png)