Stop expanding menu on mouse hover

This pull request can be previewed live here:
http://bitcoindevmenuhover.us.to/en/developer-guide

One of the issues with the single-page layout is the long menu that is sometime hard to navigate when it is much longer than the height of the page. Simplifying menus is certainly helpful, but I've realized we could perhaps improve this significantly simply by not expanding the menu on mouse :hover, and only expand the part we're currently reading.

I think this change makes the menu much easier to navigate. The menu subsections would however not be available for users with javascript disabled, since the menu is expanded using javascript when the page is scrolling (although the menu is already not awesome without javascript).