Changed the way relative velocities are calculated for docking.

I think I improved the returned values for relative velocity components (.x, .y and .z).

Before, they were returning the relative velocities in reference to the world's x,y and z, I believe.  Which meant the numbers weren't really useful for docking.

Now they should return values relative to the docking port.  

I based all of this on the way the needle movements are calculated in NavyFish's docking port.  

I've got it compiled and working.  If you DON'T want to accept this pull, because it's breaking some way someone is already using the old numbers, I'll happily rewrite it to create new defined variables that use these calculations.  I believe these were all that was missing to make a RPM docking guidance module, which I'll work on soon!  :)  

Thanks for maintaining an awesome plugin!  :)