Implement Immunities on Character

[duysqubix]

Right now immunities on character is stored on attrs property as a dictionary of lists each pertaining to an immunity in a categorty

        self.add_attr('immunity', {
            'poison': [],
            'disease': [],
            'conditions': []
        })

Change this so that the immunity is a class that stores this information, as methods can be applied to add/remove/edit current immunities.