Openfisca-core

* Variables are not added to the TaxBenefitSystem when the entities class are imported, but explicitely when the TaxBenefitSystem is instanciated.
* Metaclasses are not used anymore.
* New API for TaxBenefitSystem
* Columns are now stored in the TaxBenefitSystem, not in entities.
* New API for rerforms.
* XmlBasedTaxBenefitSystem is deprecated, and MultipleXmlBasedTaxBenefitSystem renamed to TaxBenefitSystem

* Implement cache opt out system

* Remove `build_column` obsolete function. Use `Variable` class instead.

tax_benefit_system.parameters.benefits.basic_income

* Merge pull request 382 from openfisca/fix-sum-by-entity
* The result size must be the targent entity'one

* Many updates