add scrollbar to code blocks

Love the theme, thanks for your work.

While using it, I found that my particularly long lines of code ran outside the tidy box set aside for code. 
![untitled](https://f.cloud.github.com/assets/1988384/880047/60dda678-f933-11e2-851a-9ec2c81b66e4.png)
I made a quick fix to add a scroll bar on code boxes that would otherwise overflow.