new homepage

We've talked a lot about different ways to make the homepage dynamic. Here I'm taking the opposite approach: this homepage is designed to function primarily as a first impression of Gittip that answers the most basic questions, and then leads people in one of three further directions: signing up, browsing communities, or reading about Gittip. Let's make the community and profile pages the focus of dynamic activity on the site, not the homepage.

Closes #1074.

Punchlist:

 - [x] sign in buttons under "Give" and "Receive" are way too small
 - [x] see about redoing the penny background to use one penny image + color overlays, so the pennies all line up
 - [x] fix test suite