Support keep_alive where nurse may be None

For example keep_alive<0,1>() should work where the return value may sometimes be None. At present a "Could not allocate weak reference!" exception is thrown.
Update documentation to clarify behaviour of keep_alive when nurse is None or does not support weak references.

example/example-keep-alive.ref

@@ -22,4 +22,12 @@ Allocating child.
 Releasing parent.
 Releasing child.
 
+Allocating parent.
+<example.Parent object at 0x10eb726c0>
+Releasing parent.
+
+Allocating parent.
+<example.Parent object at 0x10eb726c0>
+Releasing parent.
+
 Terminating..