Require all units to have a valid unit type.
Added damage calculation.
Added concept of hit points.

Unit sizes reduced when creatures reduced to 0 hp.
Units of size 0 no longer get a turn.
Next round begins after all units of non-zero size have acted.
Expired units disappear when hit.
Add support for animating expired units.
Added Orc animation.
Refactor image and animation loading code.