Add builder to the Curve class

[richardTingle]

Having thought about why I found the way adding control points to curves confusing I'd like to add a builder that would be used like this:

    Curve curve = Curve.builder()
            .anchorPoint(new Vector2f(0,0))
            .anchorPoint(new Vector2f(0.5f,0.5f))
            .controlPoint1(new Vector2f(0.6f,0.5f))
            .controlPoint2(new Vector2f(0.8f,2f))
            .anchorPoint(new Vector2f(1,2f))
            .build();

This example produces a straight line from (0,0) -> (0.5,0.5), then a cubic Besier curves between (0.5,0.5) -> (1,2) with control points (0.6,0.5) and (0.8,2).

The builder statically prevents end being called when not at an anchor, or controlPoint1 or controlPoint2 or nextAnchor being called at the wrong time