[demo_nodes_py] service/client mimic c++ demo behavior

[mikaelarguedas]

Decided to adress this when debugging services on Windows with @tfoote

The first commit Fixes #145
The second commit calls rclpy.shutdown() at the end of each executable. (I didnt add explicit calls to node.destroy_node() because that should have been fixed by rclpy ros2/rclpy#94)

[mikaelarguedas]

Actually ros2/rclpy#94 doesn't fix it just report the error properly. the third commit add a call to destroy_node() before shutdown so that the node handle is still valid when trying to destroy it (otherwise shutdown does shut everything down and thus the nodes handles are not valid when the Python node object goes out of scope and tries to destroy itself)

[dirk-thomas]

LGTM