Website changes

This PR includes the following:

1. The theme has been made more demure: 
    1. Top/bottom bars: *succulent* #98bc8c
    2. Side bar: *mojito* #F5FAEB
    3. Section headers: *creme brulee* #F2E3C2
    4. Code block background: *mint buttercream* #F8FFE3
    5. Background: *Emperor's new texture* - only a true PyNE developer will distinguish the pinstripes
2. The sidebar is now the primary mechanism for navigation, per our discussion at the PyNE Hack-a-thon.
3. The Sphinx cloud theme is now a dependency. Note that previously we relied on an outdated version we kept in the PyNE repo.
4. Install instructions have been improved upon, with an index page to triage new users (to direct them towards the build method that will suit them best).
5. The process for changing docs and building the website is now documented in the dev guide (closes #496).

Note that we should still consider a logo redesign, and replacing the 4 images on the front page.

![new_pyne](https://cloud.githubusercontent.com/assets/1749283/5159008/938da80a-7319-11e4-9451-17ab5680df3b.jpg)