Added an AreaControlUnit class. Its role is explicit I guess...

The players have to interact with it to get control of the map areas.
The unit classes are now in a separate package from the rest of the
world classes.
There can be more than one ControlTile for each area. They are empty.
Implemented area control attempts

To try controlling an area, place an unit on one of its control tiles.
The control attempts is done in the Environment phase. Benefits of
controlled areas are gathered when the area's control points are maxed
at the beginning of the Environment phase