Fix modifier class name generation

Should now correctly expand multiple class names or pseudo classes in a
given rule. So ".disabled.mousdown" will now accurately expand to
"disabled mousedown"

lib/kss/modifier.rb

@@ -29,7 +29,7 @@ def initialize(name, description=nil)
     #
     # Returns a CSS class String.
     def class_name
-      name.sub('.', ' ').sub(':', ' pseudo-class-').strip
+      name.gsub('.', ' ').gsub(':', ' pseudo-class-').strip
     end
 
   end