Newer example code

[gstein]

This PR provides a new revision of the sample code that has a few key features:

• construct the gmqtt.Client() outside of the run loop
• graceful shutdown of the client, and clearing of all tasks so that loop.close() does not throw errors
• switch to .run_forever() instead of the singular gmqtt task, providing an example for other tasks to run in the same event loop
• removal of the STOP event, in favor of the loop's built-in stopping mechanism