Added localGravity.decayFactor

Thanks for making this plugin.  I've added a new variable to my version called decayFactor, which will cause the orbiting object to have orbital decay.  The effect of this is that the orbiting body slowly loses it's orbit and eventually collides with the object it's orbiting.  When set to 0.0 (the default), it has no effect.  Feel free to ignore this request but I figured it might be useful. Thanks again.