Refactor to eliminate multiple inheritence.

-Move all XMLEditor methods to JSSObject.
-Move all PolicyEditor methods to Policy
-Move PolicyEditor.__init__() to new()
-Move all ComputerGroupEditor methods to ComputerGroup
-Change JSSObject.update() to handle new objects as well as existing.
-Add JSSGroupObject, move JSSGroupObjectTemplate methods to it.
-Fix inheritence for ComputerGroup, Policy
Update tests to support refactoring.