Added a live code editor based on Ace.js and rust-playpen.

Features include:
1) Sample code can be edited and run in place.
2) Results/errors are color-coded and displayed in place.
3) Lines/text with errors are highlighted in the editor.
4) Page falls-back to static code when JS is disabled.

This is what the editor looks like on a successful run:
![success](https://cloud.githubusercontent.com/assets/1480321/3165104/da18a46a-eb50-11e3-97d3-770b550671ba.png)

This is what the editor looks like on an unsuccessful run:
![error](https://cloud.githubusercontent.com/assets/1480321/3165106/dc3a4b5e-eb50-11e3-9a4a-2efd7e7ec4ca.png)