Redesign - unified styling

Changes:

- Move static files into a separate repository and add them as a submodule (https://github.com/lidavidm/sympy-web-static, I will transfer ownership of this repository once I finish working on the corresponding Live update)
- Update design to be consistent with Gamma
- Mobile design

Preview: http://lidavidm.github.io/en/index.html

Mobile:

![screenshot_2013-07-04-10-37-37](https://f.cloud.github.com/assets/327919/750560/966ce500-e4d0-11e2-969b-f13cd91e1117.png)