The top bar has been modified to calculate height based on sticky / fixed
positioning element.  Additionally, rather than jiggering the height around
manually, the ability to add custom CSS rules for a Foundation component
has been added to the base Foundation.js (addCustomRule).  It also supports
the built-in media query names (small, medium, and large) generated by the
SASS compilation.

I also updated the docs to clarify the need for contain-to-grid and fixed
to be on the same element.
Merge pull request #2395 from seantimm/2395-FixedPadding

topbar.js adding uneccessary padding to body