List enumerated types for MedicalEnumeration

As MedicalEnumeration is not currently a subclass of Enumeration, the
enumerated types are not listed. Make MedicalEnumeration a subclass of
Enumeration.

Fixes #12

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #13 from dbs/enumeration2

List enumerated types for MedicalEnumeration - adds a subClassOf relationship to Enumeration, which allows us to know that instances should be listed.