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

TODO:

- [x] fix WebKit infinite loop transition bug (automagically fixed since Bootstrap 3 switched to Mobile First)
- [x] refactor JS into a proper Bootstrap plugin (but can be improved), with proper unit tests
- [x] must be tested on all the browsers supported by Bootstrap (this is a WIP and will be unchecked whenever there's some related bug being worked out)
- [x] add proper documentation
- [x] add variant with sidebar coming from the left: `.row-offcanvas-left`
- [ ] delete temporary html file from examples