Rough port of the v3.1 Footnotes plugin to v4.0

Notes:
1. footnotes-ie.scss has not been attached. Not sure how to deal with oldIE-specific SCSS
2. Removed pseudo-focus and clip-rect mixin references. Not sure how they should be referenced in v4.0 or if they are still needed.
3. Remove inline-block Compass mixin since Compass doesn't seem to be supported and don't know what is the alternative (or if the inline-block mixin is still needed)
4. Uses .background-light in the SCSS which might be an artifact of the old grid system.