max speed (lvl) for creatures

Signed-off-by: Alexander Skjolden <alex@plastboks.net>
moved incrementSpeed method from Snake class to Creature class

Signed-off-by: Alexander Skjolden <alex@plastboks.net>
moved out food generating from GameWorld to GamePlay class

Signed-off-by: Alexander Skjolden <alex@plastboks.net>