Added jknavigable jQuery plugin (#14)

This is a very basic keyboard navigation plugin that only enables
'j' and 'k' keys for navigating up and down (vi-style).

Plugin souce: https://github.com/cakebaker/jquery-jknavigable
Added docblocks in global.js
Removed obsolete code from global.js
Merge pull request #16 from Exwebris/keyboard-navigation

Basic keyboard navigation