Make JAXBResponseParser parameterizable

By default, RestAnnotationProcessor builds the JAXBParser with the
return type of the invoked method. This, however, can make it impossible
to combine the JAXBResponseParser with the Transform annotation. The
payload of the response to be parsed with the JAXBResponseParser could
be of a different type than the output generated by the transformer
function.

This makes it necessary to configure the type for the response parser in
the JAXBResponseParser annotation and set the return type of the invoked
method to the type generated by the transformer function.
Merge pull request #1139 from nacx/jaxb-custom-class

Make JAXBResponseParser parameterizable