Removed unused classes.
Moved marshalling classes to their own package.
Forgot to move marshalling exception to its package.
Tidy up the Pet marshaller class a little bit (remove "magic" strings,
throw expections with semantics, etc.).
Refactored pets marshalling code to its own class. Also, all marshalling
related tests have been moved to the correct place.
refactored.
Implemented a new Pets marshaller based on Futures, so several Pet
objects can be marshalled in parallel.
Added more TODOs!