Charlatan

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- Add ability to uninstall fixtures (thanks to JordanB)

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- Numerous tests added, a lot of cleanup.
- Clarification in documentation.
- Remove ``load``, ``set_hook`` and ``install_all_fixtures`` shortcuts from
charlatan package.
- Remove ``FIXTURES_MANAGER`` singleton. Remove ``charlatan.fixtures_manager``
shortcut.
- Remove ``db_session`` argument to ``FixturesManager.load``.
- Add ``db_session`` argument to ``FixturesManager`` constructor.
- Remove ``charlatan.fixtures_manager.FixturesMixin``. Replaced by
``charlatan.testcase.FixturesManagerMixin``.
- ``FixturesManagerMixin`` now exposes pretty much the same method as
``FixturesManager``.
- ``FixturesManagerMixin``'s ``use_fixtures_manager`` was renamed
``init_fixtures``.

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- Add ``!epoch_now`` for Unix timestamps (thanks to erikformella)

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- Add ability to point to a list fixture (thanks to erikformella)

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- Add ability to define dependencies outside of fields through the `depend_on`
key in the yaml file (thanks to Roguelazer)

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- Fix regression that broke API. install_fixture started returning the fixture
as well as its name. (thanks to erikformella)