override default bootstrap styles for code blocks - make no-wrap with scroll bar

http://redmine.oerpub.org/issues/187

originally I wanted to change the editor to use `<code>` instead of `<pre>` or at least add another class to the `<pre>` tags to select on in the css, but couldn't figure out the aloha plugin system where all that was happening, so I just overrode all the `<pre>` styles instead.

theoretically this solution should work fine anyway, since by bootstrap convention `<pre>` is only used for code blocks anyway, but I'd still love an explanation as to how I could have accomplished the other approach just for my own education.