Implement close() functionality for meshcat viewer and zmqserver that hosts the three.js webpage

[kingjin94]

Solves #87

[kingjin94]

Tested with:

import meshcat.visualizer
vis = meshcat.visualizer.Visualizer()
vis.close()

prior to this patch vis.close() would fail and a manual killing of vis.window.server_proc would leave various processes running with ZMQbg prefix in htop's COMM / /proc//status -> Name.

Implements closing suggest, e.g., here: https://stackoverflow.com/questions/26224179/zmq-socket-gracefully-termination-in-python