Add a job structure for variant group to configurable

Add job configuration
Add VariantGroupToArrayProcessor
Add VariantGroupReader
VariantGroupReader is compatible with Mongo and send groups one by one
Process variant groups in configurables without prices
VariantGroup process normalize options price changes and base price
Merge pull request #261 from akeneo/poc/apiimport2.0

Poc/apiimport2.0