A player registers itself with a name but on the board they are stored with some ID between 1 and 15 (0 would be empty, no player at this field). Therfore it is necessary to create a relation between player name and player ID. I extended the Player.java, RegisterAckMesage.java and PrioChangedMessage.java to refelct that fact.

Added:
SnakeHeadDirection: 	static definitions to check what the content of a board-field is (snake head and which direction or body)

Changes:
Board.java: 		Added methods to check a board-field for snake or player-id
RegisterAckMessage: 	added player-ID
PrioChangedMessage: 	sends Player instead of String
Player.java: 		added an ID. this would be set by the server on registration-time. The ID is then set to the player via RegisterAckMessage