Added oveflow to correct scrolling container background bug

Sorry about that.  If a line fo code is too long for a code block, it will cause overflow and a scrollbar, but the background doesn't continue with the container and is just stuck at 100%.  Adding 'overflow:auto' corrects this by forcing a new box formatting context that encompasses the whole block. See here: http://imgur.com/a/9RQhP

Pull Request Note: Now that I look at it, this change will likely conflict with #17 pull request; however, I'm not sure #17's 'overflow:hidden' is a good idea anyway.  It should probably be 'overflow-y: hidden; overflow-x: auto;'.  This pull request is a good fix for now, but if line numbers are implemented, this topic should probably be reviewed again.