Added in 'auto' as an available value on margins

.mha replaces having a class to center block-level elements in a container. The rest are added to make this a complete set.

core/spacing/space.css

@@ -48,3 +48,7 @@ s,m,l,n = small(5px),medium(10px),large(20px),none(0px)
 .mls,.mhs,.mas{margin-left:5px !important}
 .mlm,.mhm,.mam{margin-left:10px !important}
 .mll,.mhl,.mal{margin-left:20px !important}
+.mta,.mva,.maa{margin-top:auto !important}
+.mra,.mha,.maa{margin-right:auto !important}
+.mba,.mva,.maa{margin-bottom:auto !important}  
+.mla,.mha,.maa{margin-left:auto !important}