Simplify ProductValueNorm and transfer price logic to CollectionNorm

Transfer some logic relative to ProductPrice collections normalization
from ProductValueNormalizer to CollectionNormalizer
It replaces concrete objects by interfaces or abstracts ones when it is possible
Refactor ProductValueNorm to be more extendable
Merge pull request #272 from akeneo/poc/apiimport2.0

Simplify ProductValueNormalizer and transfer price logic to CollectionNormalizer