Features page

This has been sitting on my hard drive for a while, so I figured I'd push it up.

Changes:

- Add features page. Links to documentation have not been added for each feature, but most categories feature a link to the documentation for that module. The physics category links to some example IPython notebooks.
- Highlight "About" on homepage
- Add "Why SymPy" to the homepage (it was rather empty without the features listed there)

![screenshot from 2014-03-17 18 46 54](https://f.cloud.github.com/assets/327919/2443786/805d8646-ae3f-11e3-818d-3c23bec671e2.png)
![screenshot from 2014-03-17 18 48 05](https://f.cloud.github.com/assets/327919/2443785/805b06a0-ae3f-11e3-82b9-564a2c749aa0.png)

See https://github.com/sympy/sympy-web-static/pull/4