add offcanvas layout example based on bradfrost/this-is-responsive

This is #6565 redone as a standalone example.

@mdo pointed that this should have been an example from the start (https://github.com/twitter/bootstrap/pull/6565#issuecomment-12189375)  
Later on, @fat pointed that the JS was going to be too much simple for a proper plugin (https://github.com/twitter/bootstrap/pull/6565#issuecomment-17428275)

The only thing is that the JS is still written as a plugin for ease of development, but it is placed in a comfy `/docs/assets/js/examples/bootstrap-offcanvas.js` 
Also, don't miss the TODO: https://github.com/trumbitta/bootstrap/commit/01d0cf22abb5345548b01e2e48310057d234b234#L2R55