Fix FallingGoldpiece classname
Add Ball to mruby scripting interface
Fix inheritance chain of ParticleEmitter in mruby scripting interface
Fix editor not allowing negative values for particle emitter gravity.
Allow UIDS::[] method to take *rest argument.

This makes it possible to call it with a list of UIDs now instead
of having to use a range or hackish mapping. Just do

  sprite_ary = UIDS[10, 20, 233]

and you’re done. The previous call sequences are still possible,
of course.

  sprite     = UIDS[14]
  sprite_ary = UIDS[10..20]