tested and fixed 'su-rem()' mixin

Signed-off-by: Yoannis Jamar <yoannis.j@gmail.com>
Merge branch 'master' of github.com:yoannisj/Sassy-Units

Signed-off-by: Yoannis Jamar <yoannis.j@gmail.com>

Conflicts:
	stylesheets/SassyUnits/mixins/_rem.scss