List enumerated types for MedicalEnumeration

As MedicalEnumeration is not a subclass of Enumeration, the enumerated types
are not listed. In addition to checking for Enumeration, we can check to see
if a given type is a subclass of MedicalEnumeration with an additional method
and an additional or operator.

Fixes #12

Signed-off-by: Dan Scott <dan@coffeecode.net>