[shapes] add curve shape

[teykamp]

would probably suggest using the quadraticCurveTo() function in canvas context as it only requires one control point which can be passed in as a distance from the midpoint (how far out the line curves)

[teykamp]

Reminder:
if the control point lies at t=0.5 then the apex is half way between the line connecting start and end and the control point (perpendicular to the line connecting start and end going from the center of that line). Therefore all we should have to do is to have the passed in value times 2 to get the distance the user expects from the center line start -> end