UPDATE 2015.1 - API: Added add/remove inventory methods to product and variant inventory resources.

API: Added add/remove inventory methods to product and variant inventory resources. The following endpoints have been added: PUT /api/v1/products/{id}/add_inventory PUT /api/v1/products/{id}/remove_inventory PUT /api/v1/variant_inventory/{id}/add_inventory PUT /api/v1/variant_inventory/{id}/remove_inventory All of these endpoints accept the same JSON model: {"quantity":4}