Added single generics support

This adds single generics support to JSON (See JsonGenericsTests to see how it can be used)

- Added a generator that generates the JsonReader, JsonWriter, XmlWriter and XmlReader interfaces and GWT.create calls to reduce boilerplate, especially with generics.

- Changed the "newConcreteTypes" velocity macro to use GWT.create on the Reader/Writer instead of "new Model()", because sometime the serializer needed for the embedded object is not in the same package or the default constructor is not public. This ensures the Reader or Writer is created