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.
Adjusted topbar menu caret positioning to allow for differing topbar heights
Now allowing override of -dropdown-label-bg
Merge pull request #3169 from seantimm/3169-TopBarDropdownLabelBg

$topbar-dropdown-label-bg can't be overridden.
Merge pull request #2395 from seantimm/2395-FixedPadding

topbar.js adding uneccessary padding to body