Add set_default_executor().

pep-3156.txt

@@ -217,6 +217,9 @@ Some methods in the standard conforming interface return Futures:
   executor?  Should be be able to set its thread count?  Shoul we even
   have this method?)
 
+- ``set_default_executor(executor)``.  Set the default executor used
+  by ``run_in_executor()``.
+
 - ``getaddrinfo(host, port, family=0, type=0, proto=0, flags=0)``.
   Similar to the ``socket.getaddrinfo()`` function but returns a
   Future.  The Future's result on success will be a list of the same