Added function anglediff( a1, a2 ).

returns the difference in angle between a1 and a2, quashed into the
range (-180,180].  With positve numbers meaning a2 is a bigger angle than a1.

example: anglediff( 5, 355 ) returns -10.  anglediff(355,5) returns +10.

Also squashes the result into meaningful ranges.  i.e. anglediff(365,355) handles the fact that 365 is > 360 just fine.

Important because users can't write functions yet, and KSP tends to return things like
longitude or Rotational components in stupid ways like "540 degrees".

TODO: wheelsteering should call the same thing this code does to fix the 'can't steer north' bug, but that isn't part of this commit.