Use general format for code blocks.

Code blocks by default generated to: 
```html
<pre class="brush: clojure">(+ 2 2)</pre>
```
it is specific format for Syntax Highlighter. I think it's better to have general format for such block without specific `brush:` classes. I modified it so it generates now following:
```html
<pre class="code" data-lang="clojure">(+ 2 2)</pre>
```
I think it's easier to find and parse this format from javascript than default format.