Added @gray-base variable to make @gray-{shade} reference, also allows...

Added @gray-base variable to make @gray-{shade} reference, also allows @gray-base to be modified with less#modifyVars and the vars to cascade

less/variables.less

@@ -7,11 +7,12 @@
 //
 //## Gray and brand colors for use across Bootstrap.
 
-@gray-darker:            lighten(#000, 13.5%); // #222
-@gray-dark:              lighten(#000, 20%);   // #333
-@gray:                   lighten(#000, 33.5%); // #555
-@gray-light:             lighten(#000, 46.7%); // #777
-@gray-lighter:           lighten(#000, 93.5%); // #eee
+@gray-base:		 #000;
+@gray-darker:            lighten(@gray-base, 13.5%); // #222
+@gray-dark:              lighten(@gray-base, 20%);   // #333
+@gray:                   lighten(@gray-base, 33.5%); // #555
+@gray-light:             lighten(@gray-base, 46.7%); // #777
+@gray-lighter:           lighten(@gray-base, 93.5%); // #eee
 
 @brand-primary:         #428bca;
 @brand-success:         #5cb85c;