Openfisca-core

* Include the preview API into OpenFisca-Core
* Remove internationalization (`i18n`, `Babel`)
* Use CircleCI instead of Travis

* Fix variable `source_file_path` computation.
- Make sure the tax and benefit system `location` metadata is consistent with the actual python files being loaded.

* Fix a bug introduced by 11.0.0: variables with several references could not be loaded.

* Fix a bug introduced by 12.0.0: the description of a parameter node was no longer exported in the parameter JSON.

Breaking changes

* Deprecate and remove the `fin` attribute in XML parameters.
- The end date of a parameter is assumed to be the start of the next one.
- The `<END>` indicates that a parameter does not exist anymore
* Deprecate and remove the `fuzzy` attribute in XML parameters.
- By construction, the value of a parameter is always extended towards the future.
- To indicate a likely change of the legislation, a `<PLACEHOLDER>` can be added.
* Deprecate and remove `reforms.create_item`
- New items are created directly with the method `reforms.update_items`.
* Deprecate and remove `reforms.split_item_containing_instant`
- Splitting items does not make sense in the new convention. Use `reform.update_items` to update the parameters.

Breaking changes

These breaking changes only concern variable and tax and benefit system **metadata**. They **do not** impact calculations.

* Make `tax_benefit_system.get_package_metadata()` return a `dict` instead of a `tuple`
* Make `column.source_file_path` a relative path (was absolute before)
* Make `column.url` a list instead of a string

New features

* Make `tax_benefit_system.get_package_metadata()` more robust
- Handle reforms
- Handle tax and benefit systems that are not defined within a package and/or a distribution

Technical changes

* Turn `Variable`s with a `start_date` and/or a `stop_date` into `DatedVariable`s
- This is transparent for users.