The left-section and module panes should scroll separately

@jbalsas This is the first draft of a fix for #23.

The scrolling part works good in Chrome on Win7, but that's the only testing I've done.

One issue with this is that following a link to a different module forgets the scrolling in the left-section. Is that OK? All of the ways I could think to handle that seemed like a lot of work or too clunky, so I just wanted to post this to get your thoughts.