Unobtrusive Navigation

Addresses Pivotal: https://www.pivotaltracker.com/story/show/66392266
"When in a lesson, the top nav bar gets a bit obtrusive sometimes.  Ideally we can figure out a well designed way to collapse it so users still have access to the navigation they need but in a less cluttered way."

Demo: http://mysterious-fortress-9395.herokuapp.com/ruby-on-rails/deployment

This request hides the navigation bar on scroll down and shows the bar on scroll up. Commit 47ce451 shows/hides the navigation bar when a user's cursor enters/exits the affected elements.

Manually tested on IE8+, Firefox, Safari, and Chrome. The CSS transition is not as smooth in IE8 and IE9 as modern browsers, but it is still effective in freeing screen space for lesson content. I'm open to further input and opinion. Let me know if any further testing or features should be included. 

Alex