Unit stepping

-Units now act at every game tick. (exact logic still needs to be
implemented) (GTDGame, GTDMap, GTDUnit)
-Dummy unit added as an example (check GTDUnit for movement logic).
-GTDMap::addUnit requiring a player pointer (it never did anything in
the first place).
-Changed unit positions to be double vs ints (since the game runs really
fast, unit calculations need to be more precise than integers)
-Objects can now retrieve a unit's owner via GTDUnit::getOwner();