Bootstrap styles

Applies basic styling to all of the tutorials (styles work well in mobile, too), adds a navigation bar, and coincidentally answers issue #5 - just use `<pre>` tags.

Here's a preview of what it looks like (with navbar collapsed - on a wide screen it expands):
![example](https://cloud.githubusercontent.com/assets/2932786/2734032/d7e27d6e-c64e-11e3-8875-5602b470a969.png)