Improved the JSON patient export format

This is work in progress.
Has the basic structure and interfaces setup.

- Created a separate package (org.phenotips.data.internal) for classes that hold patient data. Each class is an implementation of a new interface (PatientDataSerializer).
- Each of those classes implements a function that reads data from the wiki, exports json, and imports it.
- Import function is not implemented yet.
- So far, each class ROUGHLY corresponds to a class in xwiki (PatientClass, OntologyVersionClass).
- The idea is that the PhenoTipsPatient class would store all of its data in classes implementing PatientDataSerializer, rather than storing and dealing with data itself.