Make the point dragging demo use websockets

[goatchurchprime]

Simple implementation available here:

https://websockets.readthedocs.io/en/stable/intro.html

Then have to commit the code here.

Other features:

(1) make better move PID control function

(2) Plot on page to scale and at the right orientation for the two motors

(3) Produce javascript loop that steps through and sends XY commands in realtime using: http://dmitrybaranovskiy.github.io/raphael/reference.html#Raphael.getPointAtLength based on an SVG path sequence.
(this is a replacement for G-code.)

As there is feedback of the real position via the BB websocket, the progression along the path can speed up or slow down accordingly. Thus implementing the proposed adaptive control in a very few lines of explicit code. If we hang a weight onto the gondola it should respond immediately by going slower.

[goatchurchprime]

nearly happening but for a problem getting eqeps to work under new os

[goatchurchprime]

The dragging thing now works. The code is at: https://bitbucket.org/goatchurch/bbhquad/src/4705c041c5b5df719388c7ff848c7d73ed17de3f/polarcode/?at=master