Off canvas menu implementation #6
Description
When screen size is less than 480px, we need to switch to an off canvas menu.
I've separated navigation from header in preparation of this menu implementation.
As a guideline, we should follow what have been accomplished here : http://dbushell.github.io/Responsive-Off-Canvas-Menu/step4.html by reading this article : http://coding.smashingmagazine.com/2013/01/15/off-canvas-navigation-for-responsive-website/