Merge DummyCountry and boilerplate

[MattiSG]

As a new Producer,
I can start coding my tax and benefits system with an always up-to-date boilerplate,
So that I don't learn an outdated syntax

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Merge https://github.com/openfisca/openfisca-dummy-country and https://github.com/openfisca/country-template/ and use the result as a test base for openfisca-core.

[MattiSG]

Import and editorialise the DummyCountry into the Boilerplate, demonstrating standard and advanced uses, adding comments linking to the doc, and suggesting future-proof architecture (i.e. using folders and showing that several files can be used).

[MattiSG]

[MattiSG]

Based on openfisca-france:

• Add .gitignore with proper defaults (.DS_Store, IDEs, pyc…).
• Add Makefile with proper test (and lint?).
• Add CONTRIBUTING.md.
• Add LICENSE.AGPL.txt.
• Add .github/ISSUE_TEMPLATE (from openfisca-core).
• Add .github/PULL_REQUEST_TEMPLATE (translate from openfisca-france).
• Add README.md: be clear about that repo does, but expect forks not to modify it properly.
• Add CHANGELOG.md: use the format of france.
• Add MANIFEST.in.
• Add .circle.yml and set up sane testing defaults.
• Add setup.py.
• Move tests to openfisca_country_template/tests and update the version check code.
• Move param.xml to parameters/*.xml.
• Move entities.py to entities/__init__.py.
• Move scenarios.py to entities/relations.py.
• Remove assets.
• Remove conf.
• Remove decompositions.
• Remove extensions.
• Remove i18n.