Updated Order and Cart to be rest resources.
Added a Queryable interface that can be used to indicate that a Rest Resource supports querying.
Added getValueAsString() method to Id.
Removed spurious warnings.
Moved IdTypeAdapter as an inner class of Id (Id.GsonTypeAdapter).
Created a internal.util package to keep commonly used classes such as Preconditions.
made common base classes such as ParamMap, ContentBody public.