[Compass Core] New mixin: +opacity(amount). Provides cross-browser css opacity.

Amount should be between 0 and 1, where 0 is transparent and 1 is opaque.

frameworks/compass/stylesheets/compass/utilities/_general.sass

@@ -4,3 +4,4 @@
 @import general/tag_cloud.sass
 @import general/inline_block.sass
 @import general/hacks.sass
+@import general/opacity.sass

frameworks/compass/stylesheets/compass/utilities/general/_opacity.sass

@@ -0,0 +1,15 @@
+// expects a number from 0 to 1
+=opacity(!opacity = 1)
+  :opacity= !opacity
+  :-moz-opacity= !opacity
+  :-khtml-opacity= !opacity
+  :-ms-filter= "progid:DXImageTransform.Microsoft.Alpha(Opacity=" + round(!opacity*100) + ")"
+  :filter= "alpha(opacity=" + round(!opacity*100) + ")"
+
+// Make an element completely transparent.
+=transparent
+  +opacity(0)
+
+// Make an element completely opaque.
+=opaque
+  +opacity(1)