Adding support for configurable label font colors.

Font colors we're previously hardcoded within _labels.scss.

I've added two new variables named $label-font-color and
$label-font-color-alt.

Since form labels used the variable prefix $label-, I've renamed these
to $form-label-
Merge branch 'label-font-color' of git://github.com/lukebussey/foundation into lukebussey-label-font-color
update changelog