add 'clock' example

[gbrault]

clock.py and index_clock.html show an example where:

• the client opens the ws channel
• the server send every seconds a time stamp
• the client display it

The client clicking on the close button is closing the ws channel

[miguelgrinberg]

Thanks! I've merged this with some minor changes for consistency.

[gbrault]

You are welcome! Thanks for your work @miguelgrinberg