Add experimental off-canvas layout

I don't think this is going to ever be accepted, but I hope it could be a nice beginning to play with.  Just read on...

Based on:
https://github.com/bradfrost/this-is-responsive/blob/gh-pages/patterns/layout-offcanvas-right.html

It lacks a nice transition effect due to a strange bug happening
only in Chrome at certain viewport sizes. The transition is there, but it's commented out.

Other gotchas: 
 * it works only with a sidebar coming in from the right
 * the required javascript is ugly and written in the HTML file
because I don't feel comfortable enough to figure out how to write
a noob-level plugin for Bootstrap.