Include new class StructureType in module
atlantis.gamedata.rules documentation.
New class ItemUnit in atlantis.gamedata.item module to hold reference to
items in units.
Fix minor bug that happens when dragging is so fast that it's fired
before starting position is saved.
New module atlantis.gamedata.skill
Definde json serialization methods incrementally for derived classes.

That means that a derived class should call its parent serialize method
and then add its own data to the dictionary.
Fix a bug with tests on ItemUnit class.
Rewrite ReportConsumer.region_unit method to use ItemUnit, Skill,
SkillLevel and SkillDays classes instead of plain dictionaries.