function RelativePlugin() {
		throw("RelativePlugin plugin cannot be instantiated.")
	}
The RelativePlugin for TweenJS enables relative numeric values for tweens. Install using:

	RelativePlugin.install();

Once installed, you can pass in relative numeric property values as strings beginning with "+" or "-". For example,
the following tween would tween the x position of `foo` from its initial value of `200` to `50` (200-150), then to
`125` (50+75).

	foo.x = 200;
	Tween.get(foo).to({x:"-150"}, 500).to({x:"+75"}, 500);

@class RelativePlugin
@constructor