Gidgethub

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- Introduce :mod:`gidgethub.tornado` to support
`Tornado <http://www.tornadoweb.org/>`_ (thanks to
Matthias Bussonnier and A. Jesse Jiryu Davis for the PR reviews).

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- The default value for the *data* argument of :meth:`gidgethub.abc.GitHubAPI.put`
was changed from ``""`` to ``b""``.
- All type hints were removed (due to mypy not supporting yield in an async
function, they were not being tested as being valid).

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- Renamed ``gidgethub.abc.GitHubAPI._sleep()`` to
:meth:`~gidgethub.abc.GitHubAPI.sleep()` to make the method public.
- Renamed the "test" extra to "tests" and added the "dev" extra.
- Introduced the :exc:`gidgethub.RateLimitExceeded` exception.
- Methods on :class:`gidgethub.abc.GitHubAPI` no longer automatically sleep when it's
possible that the call will exceed the user's rate limit (it's now up to the
user to prevent from going over the rate limit).
- Made the ``[treq]`` install extra depend on ``Twisted[tls]``.

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- :meth:`gidgethub.sansio.Event.from_http` raises a :exc:`gidgethub.BadRequest` of ``415``
instead of ``400`` when a content-type other than ``application/json``
is provided.
- More robustly decode the body in :meth:`gidgethub.sansio.Event.from_http`
(i.e. if the ``content-type`` doesn't specify ``charset``, assume ``UTF-8``).
- Changed the signature of :class:`gidgethub.sansio.Event` to accept
:data:`typing.Any` for the *data* argument.
- Fixed signature verification.

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- Introduced ``gidgethub.treq`` (thanks to Cory Benfield).

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Initial release.