Sticky navigation option

This PR introduces a theme option to enable "sticky" navigation. Demo: http://youtu.be/6tBrRIbxgHQ.

Features:
 * Disabled by default, so all existing installs are unmodified
 * User can turn on/off from ``conf.py`` via ``sticky_navigation`` boolean option
 * If nav bar is taller than window, stick nav is turned off. This prevents "unreachable" spots in the nav bar. See YT video for a demo.