Support for the new JSON PostgreSQL data type

Hi!  I believe the attached changes enable this package to work with the new JSON data type available since PostgreSQL 9.2.  I’ve included the type in the generators for the built-in types and for static type information, and in a separate commit, the generated files — in all, the changes are quite trivial.  The tests ran fine on GHC 7.4.2 and PostgreSQL 9.3beta1.