Aiohttp-things

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**Added**

** added ``views.Jinja2Mixin`` (need install ``aiohttp-jinja2``);
** added ``views.JSONMxixin``;
** added ``views.ListMixin``;

**Changed**

* ``views.PrimaryKeyMixin`` allocated in a separate mixin;
* ``views.ItemMixin`` inherited by ``views.ContextMixin`` and
``views.PrimaryKeyMixin``.

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**Changed**

* ``views.PrimaryKeyMixin`` renamed to
``views.ItemMixin``.

**Added**

* Added ``views.PrimaryKeyMixin`` synonym for ``views.ItemMixin``.

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**Changed**

* Replace ``aiohttp.abc.AbstractView`` to ``views.ContextMixin`` in parent
classes of ``views.PrimaryKeyMixin``.