Allow multiple context managers in one with statement, as proposed
in http://codereview.appspot.com/53094 and accepted by Guido.

The construct is transformed into multiple With AST nodes so that
there should be no problems with the semantics.
diff --git a/Include/graminit.h b/Include/graminit.h
index 38b4dac..6df6509 100644
--- a/Include/graminit.h
+++ b/Include/graminit.h
@@ -42,7 +42,7 @@
 #define for_stmt 295
 #define try_stmt 296
 #define with_stmt 297
-#define with_var 298
+#define with_item 298
 #define except_clause 299
 #define suite 300
 #define testlist_safe 301