Call beginPath() before drawing each segment

For some reason, this makes the picture appear much clearer.

This will also enable us to incrementally draw the lines (see issue #5)
Add animation

This is done via setInterval().  Instead of having a double for loop in the
draw() method, draw() simply calls setInterval(), which in turn calls the
callback drawLine() at set intervals of time.

A "speed" parameter is now included in the Paley drawer object.  This is a
misnomer, since the higher the number the slower the graph will be drawn.

Fixes #5
Simplify sample.html

From now on, one Paley object per canvas!
Merge branch 'animate' into paley-master