Add ext type for https://github.com/gofrs/uuid uuid type.
Change test and README from github.com/satori/go.uuid to
github.com/gofrs/uuid. The reason is due to this issue:
https://github.com/satori/go.uuid/issues/73. This was taken on board and
fixed in the community project of gofrs. The gofrs implementation has
the same interface as the original.
This adds PostgreSQL numeric mapping to and from
github.com/shopspring/decimal.
Makes pgtype.NullAssignTo public as external types need this functionality.
Begin extraction of pgtype testing functionality so it can easily be used by
external types.