Links now rendered properly when taller than they are wide

Each link between nodes is drawn using a bezier curve from the right-hand side of the From node to the left-hand side of the To node.  The stroke width of the bezier curve corresponds to the amount of flow.

If a bezier curve has a wide stroke width and slopes up or down steeply, it sweeps out too much area.  Here's a picture of the problem: http://s27.postimg.org/ifhkwvddv/render_problem.png

I fixed the problem by placing a maximum on the widths of the Bezier curves.  Large links are now drawn using several smaller adjacent bezier curves.

This is a breaking change: existing code setting each path's stroke width to the link's `dy` property will now need to use the link's `strokeWidth` property.