hub.Waiter: add __call__ (link protocol)

gevent/hub.py

@@ -411,6 +411,12 @@ class Waiter(object):
             finally:
                 self.greenlet = None
 
+    def __call__(self, source):
+        if source.exception is None:
+            self.switch(source.value)
+        else:
+            self.throw(source.exception)
+
     # can also have a debugging version, that wraps the value in a tuple (self, value) in switch()
     # and unwraps it in wait() thus checking that switch() was indeed called