Openfisca-web-api

* Add documentation and example files to deploy in production.

* Make the script `openfisca-serve` work with any country
- Before, it was always loading `openfisca-france`
- Now, the country package (as well as reforms and extensions) can be provided through arguments
- If no country package is provided, an auto-detection is attempted

For more information about how to use the script, run `openfisca-serve -h`.

* Bugfix: in development mode, fix the path of the country package. This impacts the `parameters` endpoint.

* Accept other countries than OpenFisca-France and OpenFisca-Tunisia.
* The endpoint `parameters` now removes the `description` key from the JSON response, instead of an empty string,
when nodes of the legislation XML have no description.

* Fix the condition to detect if the country package has decompositions:
* Load decompositions only if `tax_benefit_system` has a `decomposition_file_path` attribute.

* Rename variables to make the tests pass.
* Remove the dependency constraint to France.