Add MercadoPago payment method!

I think its neccesary to include in opencart by default MercadoPago in the payments methods.

There is only one important reason for this. In Latin America its the biggest way to send money, buy or sell items. A lot of people not use opencart because of this.

They have an api and the mod done. Its only add to the defauel package https://github.com/mercadopago/cart-opencart