support pgx/stdlib to drive PostgreSQL.

[IsvaraWoo]

Someone like me connect PostgreSQL with jackc/pgx.
jackc/pgx is not compatible with database/sql, but it given a database/sql compatibility layer by jackc/pgx/stdlib.
jackc/pgx use "pgx" as driver indicator, so connect PostgreSQL with pgx like below:

db, err := dbr.Open("pgx", "user=pguser password=secret dbname=mydb sslmode=disable", nil)
if err != nil {
	log.Fatalln(err)
}
...

[taylorchu]

thanks!