adding off-canvas navigation

updating css to have a responsive off-canvas navigation for small screen devices, 
all required html / js code are pushed into the [requirejs](https://github.com/jrburke/requirejs) repo.
![requirejs-docs-off-canvas](https://f.cloud.github.com/assets/559002/1699185/7fb39548-5f87-11e3-9487-98652ed624ed.jpg)