Add schema graph

[qdequele]

This pull request improves the schema scraper by taking into account the @graph object, which means the schema will not be describing one item (Product, Article) but multiple items. Taking this into account, it allows to create a new document per item and not one document for every item on the page.

By doing this, I also saw that sometimes the index creation was not taking into account the right primary_key. I simplified the process to always send the primary_key (given on the config or uid) within the index creation.

[qdequele]

@bidoubiwa, why do I have one test not passing? Do you know?