Provides access to a curve's control points

I've added a `getControlPoints` instance method which generates and returns `[{x,y}, {x,y}]` for the curve. I like getting the points rather than a flat array of arguments as it's more semantic; I'm generating it each time to prevent the developer from editing it and thinking it'll have an effect. I'm assuming that it won't be called often enough to matter that it's generating new each time.

Adds tests for getControlPoints and toString.

Thoughts?