Added Character class.
Re-structured the entity hierarchy for differences between living entities
and non living entities.
The hierarchy as it stands is: Entity-->Character-->Player, Enemy.
Made every necessary change to incorporate the hierarchy re-structuration,
such as EntityManager changing into CharacterManager.

Commands now have an attribute for it's string representation, instead of
duplicating a switch search for the corresponding string.

Created a static ErrorStack class, which can be called to add visible
String errors to the stack for later review.

Signed-off-by: Kevin <nivek_et@hotmail.com>