#3 Add read objects for inventory domain

[ronan-gloo]

This a test / expected implementation about how to access to API "domains".

Inventory is implemented as an example here :

• One Domain class, as an entry point, where all domain possibilities are accessibles
• One Resource class, which represent an particular inventory hal entity
• One or more operations classes (here: InventoryUpdate), in order to modify the domain state

Documentation can be found at docs/inventory.md. Exposed methods does not aims to be exhaustives, but try to find a consistent and self documented way to expose our Api.