Update to bootstrap 3.1.1 and change theme a bit

I think angularjs.org seriously needs some love.
Inspired by angulardart.org I updated bootstrap to 3.1.1 and changed the theme a bit.
Demo: http://fiws.github.io/angularjs.org/

These changes include:
* Optimized the page for mobile
* New hero
  * Fork me on github badge
  * Red download button
  * Download dialog integrated in hero
* Changed the navbar theme to white and moved the menu entries to the right (like on angulardart.org)
* New layout for code demos
  * Preview container no longer a default bootstrap well
  * Chevron (>) indicating code execution
* No more default bootstrap modals
* Changed fonts
 * Roboto for headings
 * Source Sans Pro as default
* Added html5shim to support older browsers
* Small changes like
  * Added a horizontal line between the code examples (copied from angulardart.org)
  * Removed dummy youtube video pictures and replaced them with "watch tutorial" buttons
  * Using vector graphics for both logos (falling back to png)
  * Code cleanup

I hope you like these changes. I'm open for criticism or suggestions. :)
(And sorry for my possibly bad English)

## Screenshots
![screenshot from 2014-02-16 19 13 19](https://f.cloud.github.com/assets/3409958/2180887/96da38c6-9736-11e3-9e13-1bc3f7183763.png)
![screenshot from 2014-02-16 19 14 51](https://f.cloud.github.com/assets/3409958/2180889/aeaa4cfc-9736-11e3-8340-a222c682f081.png)
![screenshot from 2014-02-16 19 16 17](https://f.cloud.github.com/assets/3409958/2180891/b617daea-9736-11e3-93de-a6c79dab0650.png)

## Mobile
(Sorry for high resolution)

![screenshot_2014-02-16-20-03-16](https://f.cloud.github.com/assets/3409958/2181006/718233c8-973e-11e3-9786-dac480b16140.png)
![screenshot_2014-02-16-20-03-59](https://f.cloud.github.com/assets/3409958/2181007/751ff9f2-973e-11e3-8259-604ae5a42f9d.png)
![screenshot_2014-02-16-20-05-24](https://f.cloud.github.com/assets/3409958/2181009/790ebf12-973e-11e3-8a6f-6c610c7b8d9d.png)
![screenshot_2014-02-16-20-06-29](https://f.cloud.github.com/assets/3409958/2181011/7b842c14-973e-11e3-80ef-8010fdcffeb7.png)