Add support for foreign keys

[RichardBradley]

As far as I can see, without using execute, there is no support for foreign keys.

[djung-fodessa]

I'd like this also. It isn't too pretty to be writing:

$this->execute("ALTER TABLE products ADD CONSTRAINT fk_products_customers1 FOREIGN KEY(id_customer_merchant)"
                ." REFERENCES customers(id_customer) ON DELETE NO ACTION ON UPDATE NO ACTION");

I'll add it if I have time, though that may never happen unfortunately.