fix(checkbox): use a more stable top positioning

Previous top positioning was using fixed 2px. This could not look good if @line-height were changed; instead, we use a rem value, based in @line-height and @checkbox-size variables.
style(frontkit.less): separate forms category from elements category
feat(forms): allow checkbox and radios to be used within a .form-checkbox/.form-radio
docs(forms): use checkbox directive instead of plain checkbox
chore(watch task): watch changes on test specs
feat(radio directive): created radio directive
docs(checkbox/radio): added docs for radio together with checkboxes docs