Add UIDS::cache_size and UIDS::cached_uids
Add Spika to mruby scripting interface
Add red spika enemy.

Just wanted to test out how to add enemies. And a small & fast spika
was missing IMHO.
Add Spikeball to the mruby scripting interface
Add thromp to mruby scripting interface
Add StaticEnemy to mruby scripting interface
Add turtle to mruby scripting interface
Add TurtleBoss to mruby scripting interface.

With regard to issue #15, this means we now can script all existing
enemies.