[WIP] Basic styling changes

Changes:

- Fonts: use Source Sans Pro as body font, Gentium Basic as website header ([development version of Gamma](http://sympy-gamma-li.appspot.com) uses Open Sans/Gentium; do you like the Gamma font better?)
- Minor adjustment of spacing
- Get rid of background gradient
- "Documentation" links directly to docs.sympy.org

Other ideas:

- Change right sidebar to match Gamma's "card" visual style?
- Change header to match Gamma's (white on green background); current header doesn't look too attractive
- Perhaps tone down those drop shadows...
- There is a lot of green on green on green, which is probably a bad idea
- Make features on homepage link to Gamma/Live examples
- Have "Screenshots" link to Live examples
- Add searchbox for Gamma

![screenshot from 2013-05-02 17 17 01](https://f.cloud.github.com/assets/327919/457094/cb65ab0a-b386-11e2-90e4-aa2aaf7239d3.png)