Set default Simulation.steps_count to 1. Add new method Simulation.new_holder.
Accept multiple decomposition formats.
Move attribute formula_constructor to column.
Move attribute _func to column.
Always create a new state, to avoid multithread errors.
Rename method sum_by_roles to sum_by_entity.
Rework simulation initialization.
Simplify Holder.compute.
Cache legislation JSON instead of parsing it, each time a tax benefit system is created.
Add method to compute the dependencies of columns.
Add a version

@ervaiart: this is needed bu openfisca-qt but should we put it here ?
Remove level from graphs.
Add to_json method to holders and formulas.
Revert "Add a version", because infos needed only by OpenFisca-QT don't belong to core.

This reverts commit f3d262ab699087c0cb5e2fedf0dbf595a41ea72d.
Add consumers to variables.
Don't fail when a column has no consumer.
Merge branch 'master' of https://github.com/openfisca/openfisca-core