Make off-canvas compatible with fixed position elements

Given this component wraps the whole page, and the way [transforms mess with position:fixed elements](http://meyerweb.com/eric/thoughts/2011/09/12/un-fixing-fixed-elements-with-css-transforms/), we have to remove this default transform.

Still need to investigate changes to this: https://github.com/99designs/asimov-off-canvas/commit/ce50c962aaefec4e1b6e454241337b602efe5fee

/cc @dennisroethig @xzyfer 
Nitpick but if you have an else branch it's generally more readable to just have `if (foo) else` rather than `if (not foo) else`