Added missing Decal.setRotation method.

http://stackoverflow.com/questions/24632095/how-to-rotate-decal-over-more-then-one-axis-at-moment

It looks like currently the only way to set a rotation on all axes at the same time is via a workaround like this:

    decal.setRotationX(0);
    decal.setRotationY(0);
    decal.setRotationZ(0);
    decal.rotateX(angleX);
    decal.rotateY(angleY);
    decal.rotateZ(angleZ);

Or the shorter but more ugly version:

    decal.setRotationX(angleX);
    decal.rotateY(angleY);
    decal.rotateZ(angleZ);

That's why I added a `Decal.setRotation(...)` method which can set all angles at once.