Added methods to find the current active section in the site tree.

Because the only way to detect the active menu item is to compare
its post ID to the current page's post ID, if the current page is
in the menu twice, the last instance of it in the menu is going
to be marked as the "active" page.
Changed 'active' class to 'active-page' and added 'active-section' class.
Updated section nav styles.
Merge branch 'develop' of git://github.com/johnshopkins/machado into develop
Gulped for staging.