TWEAK: Add New API Methods

We’ve got some people needing to walk up the shipping stack on an order to get to the shipping code field and some working with gift certificates, so we are working with them to empower their custom applications.

• ShippingProvider_GetByName added
• ShippingProvider_GetByCode added
• GiftCertificate_GetByCode changed to allow searching all gift certificates, regardless of what customer has it assigned.