PIM-2156 : add basic ORM field sorter
PIM-2156 : add product sorter extension
PIM-2156 : not apply ORM sorter for product grid
PIM-2156 : add applySorterByAttribute method in MOngo product repository
PIM-2156 : register sorters in product sorter extension
Merge branch 'master' into mongo/grid-sorter
Fix CS
PIM-2156 : add an abstract sorter extension
PIM-2156 : ORM sorter extension inherits from abstract
PIM-2156 : product sorter inherits abstract extension
PIM-2156 : add abstract pager extension
PIM-2156 : ORM and product pager inherits from abstract extension
PIM-2156 : extensions configuration cleaning
PIM-2156 : add an abstract extension
PIM-2156 : ORM and product filter extensions now inherit from abstract
extension
PIM-2156 : update filter extension configurations
PIM-2156 : CS fixer
PIM-2156 : update the change log
Merge pull request #833 from akeneo/mongo/grid-sorter

Grid sorter & extensions re-working (MongoDB ODM support)