Add fr relative size unit

defined in the CSS3 Grid Layout Module

http://www.w3.org/TR/css3-grid-layout/#fr-unit

Author: sabberworm

lib/Sabberworm/CSS/Value/Size.php

@@ -5,7 +5,7 @@
 class Size extends PrimitiveValue {
 
 	const ABSOLUTE_SIZE_UNITS = 'px/cm/mm/mozmm/in/pt/pc/vh/vw/vm/vmin/vmax/rem'; //vh/vw/vm(ax)/vmin/rem are absolute insofar as they don’t scale to the immediate parent (only the viewport)
-	const RELATIVE_SIZE_UNITS = '%/em/ex/ch';
+	const RELATIVE_SIZE_UNITS = '%/em/ex/ch/fr';
 	const NON_SIZE_UNITS = 'deg/grad/rad/s/ms/turns/Hz/kHz';
 
 	private $fSize;