PIM-2317 : add a product fields transformer
PIM-2317 : add a datetime transformer
PIM-2317 : use the field transformer to delegate part of the hydration
PIM-2317 : add values transformer
PIM-2317 : add option transformer
PIM-2317 : update product hydrator touse new transformers
PIM-2317 : add family transformer
PIM-2317 : use family transformer in product hydrator
PIM-2317 : add completeness transformer
PIM-2317 : add completeness transformer spec
PIM-2317 : use completeness tranformer in product hydrator
PIM-2317 : add groups transformer
PIM-2317 : use groups transformer in product hydrator
PIM-2317 : fix CS
Merge pull request #953 from akeneo/mongo/product_hydrator_specs

[WIP] Product hydrator re-working and specs (MongoDB support)