HTML Export: Add some default classes for text alignment

Proposal by Sebastian Rose.

lisp/org-html.el

@@ -126,6 +126,9 @@ not be modified."
   .target { }
   .timestamp { color: #bebebe; }
   .timestamp-kwd { color: #5f9ea0; }
+  .right  {margin-left:auto; margin-right:0px;  text-align:right;}
+  .left   {margin-left:0px;  margin-right:auto; text-align:left;}
+  .center {margin-left:auto; margin-right:auto; text-align:center;}
   p.verse { margin-left: 3% }
   pre {
 	border: 1pt solid #AEBDCC;