#6 curve builder

[richardTingle]

Adds a builder for curves that can be used like

 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();

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