Fix for $topbar-bg assuming only solid color for topbar background, now allows gradients, images, transparency, etc

modified $topbar-bg definition to split color definition from background definition, replaced all references to $topbar-bg with $topbar-bg-color, added new variable for $topbar-dropdown-label-bg

Fixes https://github.com/zurb/foundation/issues/2111 by allowing any valid background CSS to be set for $topbar-bg, including gradients, images, transparency, etc.