#44 Subclass support

[Michael]

Provide some support for subclassing. This would be an optional element in the xsd for CpoClass and just a text box on CpoUtil. When the java source was generated for a class, if this was set, the class would "extends x" where x is the superclass.

Potentially, we should modify the equals(), hashCode() and toString() methods to call super() as part of the implementations.