offcanvas wraps set to 100% height

Fixes the clipped Offcanvas issue that occurs when content height is low (or there's no content loaded yet). Seen others struggling with this [here in the Foundation forum](http://foundation.zurb.com/forum/posts/273-full-height-off-canvas) and [on stackexchange](http://stackoverflow.com/questions/20158607/foundation-5-off-canvas-full-height-of-device/20290707).
This fixes the issue for me, but html and body heights still need to be set to 100% for it to work. I might have missed something, but this seems like the correct solution to me.