Game Save / Object Ownership

Added a save system to the game. Currently, this queries all item
managers for their items' data, which is passed back in the form of a
TiXmlElement and saved to an XML file. Currently no data on which level
the player is on is saved.

Added an ItemManager base class for item managers (E.g. Component
managers) to inherit from. All game items created during gameplay
(components, EncounterGroups, etc..) are now owned by their respective
item managers, which are responsible for creating and deleting these
objects.