Favour Externalizer method implementations over @Marshalls annotation values

Enhance Externalizer interface with methods for getTypeClasses(), getTypeClassNames() and getId() in favour of annotations.

The reason for doing this is twofold:

• The use of annotations makes it difficult for other apps to abstract the externalizer framework.
• Runtime errors not caught at compilation time could arise from code like this where X and Y have no relationship:

@Marshalls(typeClasses = Y.class, id = ExternalizerIds.XExternalizer)
public class XExternalizer implements
Externalizer<X> {

In particular, code like this would maintain guarantees between classes externalized and return of readObject():

List<Class<? extends T>> getTypeClasses();