Created Player class
Users can only pick up Entities belonging to them
Fixed a bug where Entities would disappear if they moved to their original Tile
Send messages to the server when a Player buys a PeaceEntity
Improved networking
PeaceEntities have a cost to buy. Players have money.
Separated Units HashMap from UnitsForMarket HashMap.
(UnitsForMarket are the units available to be added to the Market)