added css3 background-size mixin

lib/compass/frameworks/compass/stylesheets/compass/css3/_background_size.sass

@@ -0,0 +1,14 @@
+// Background Size
+// Set the size of background images using px, width and height, or percentages
+//  - percentages are relative to the background-origin (default = padding-box)
+//  - mixin defaults to "100% auto"
+
+//  override constants to change defaults
+!default_background_size ||= 100% "auto"
+
+=background-size(!size = !default_background_size)
+  background-size= !size
+  -webkit-background-size= !size
+  -o-background-size= !size
+  -khtml-background-size= !size
+  -moz-background-size= !size