Add methods to manipulate shippers

[bglamadrid]

Sales may specify a certain Shipper entity. But these must exist before being stated at checkout time. And currently they are missing their API resources for listing, creation, edition and deletion.

After next major version comes up, these API resources should be created:

• GET /data/shippers
• POST /data/shippers
• GET /data/shippers/{id}
• PUT /data/shippers/{id}
• DELETE /data/shippers/{id}

And obviously, Shippers should have an id property to be able to follow this flow.