Make API consistent

[astagi]

Try to make no differences between properties and objects

[edoaxyz]

Unfortunately the rendundancy happen only in the Product's categories and tags. Other properties that are linked to other objects have some differences between them. An example are Product's attributes, which can be referenced to a Product Attribute object, but can be also a new one created and available only for that Product. Another example is related to the Orders: they contains the informations about the ordered Product but if this last one will be updated, the one in Order won't, so there would be some discrepancy. I would think to add some methods to these "special" properties (eg. Product Attributes) in order to link them to the actual objects, if they exist. Check this