Fixed tab-bar with fixed off-canvas

This is a popular issue -
http://foundation.zurb.com/forum/posts/547-off-canvas-with-fixed-top-bar
http://foundation.zurb.com/forum/posts/310-tab-bar-fixed

I've added a test page at /docs/offcanvas-fixed.html (sibling to /docs/offcanvas.html test page)

Did some consolidation of the recent changes by @winghouchan - would be great to get some of your feedback.

I've added a window resize listener to offcanvas.js, since vh units are not widely supported yet (SOOON!!!). It only gets activated if the new `.off-canvas-fixed` container is present.