Add Ping's enhanced singleton decorator.

pep-0318.txt

@@ -270,7 +270,12 @@ some examples of use.
    ::
 
        def singleton(cls):
-           return cls()
+           instances = {}
+           def getinstance():
+               if cls not in instances:
+                   instances[cls] = cls()
+               return instances[cls]
+           return getinstance
 
        class MyClass [singleton]:
            ...