Py-css-styleguide

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* Added automatic enable references mode, close 1;
* Fixed Sass helper function ``to-string`` for empty list, close 2;
* Changed ``list`` and ``string`` structures so they can be empty, close 3;
* Internally use ``collection.OrderedDict`` instead of simple dictionnary in parser and
serializer, close 4;
* Fixed code quality issues with Flake8, close 5;

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* Added Sass function ``floor-number-items()``;
* Removed ``flat`` property in favor of ``structure`` to allow other structure modes;
* Added new structure mode ``list``;
* Added new structure mode ``string``;

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* Added documentation with Sphinx;
* Changed ``Manifest.load()`` so it also accepts a file-like object;
* Added test for Sass mixin helper using Boussole;

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* Added ``to_json`` method to Manifest model;
* Fixed some code quality issues;

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First commit with a basic working version.