Add support for comparing JSON column on PostgreSQL driver

[alileza]

In the meantime, comparing JSON column for db/sql postgres driver would throw an error. It would be nice to have a way, to make it possible.

Then "tomato-mysql" table "customers" should look like
        | customer_id | name    | metadata                   |
        | 1           | cembri  | {"ip_address":"127.0.0.1"} |