Add support for relative values

[jverkoey]

trafficstars

Values passed to the animator are currently absolute. It would be nice to also be able to provide relative values.

// Move a view 50 points down.
    [animator animateWithTiming:timing
                        toLayer:view.layer
             withRelativeValues:@[ @0, @50 ]
                        keyPath:MDMKeyPathY];

This feature should take care when implementing support for shouldReverseValues.