Moved entity data files into their own folder, EntityData.

Level data will probably be organized into its own folder, LevelData. However, I am still unsure as to whether entity-specific data should be level-specific (ex. a tank of EnemyTank1 would have a different movement speed on each level), or certain entity types should be used on certain levels (ex. rather than use a slow EnemyTank1 on Level 10, use a faster EnemyTank14).