Malaffinity

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* Happy birthday to me.
* Correct incorrect information in the ``NoAffinityError`` docstring, which
stated that the minimum shared rated anime threshold for calculating affinity
was ten, when it's actually eleven.

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* Add the ``conventions`` section to ``extras_require``, so conventions
dependencies can easily be installed if needed.
* Add the ``travis`` section to ``extras_require``, so Travis can easily
install any additional dependencies it needs.
* Rename the ``doc`` and ``test`` sections in ``extras_require`` to ``docs``
and ``tests`` respectively.

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* Create the ``Affinity`` namedtuple, and return that instead of a normal tuple
when calculating affinity.
* Rename the "their_list" var in ``MALAffinity.calculate_affinity`` to
"user_list". It's an improvement, but still a bad var name.

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* Tidy up the MALAffinity ``__repr__``, and dynamically specify the class
name, in case the class needs to be renamed (which it probably won't).
* Use double backticks instead of a single backtick in the ``calculate_affinity``
function's docstring.
* Add a one-off comparison function (+ docs).
* Move the "shared rated anime threshold" check to the ``calculate_affinity`` method,
as a comparison can be done no matter how many shared rated anime two users share.
* Add the error/warning codes to ignore next to the `` noqa`` comments.
* Rewrite the contributing page in the docs, and create ``CONTRIBUTING.rst``
to link to it.

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* Use ``Xinil`` and ``Luna``'s usernames to demonstrate how MALAffinity
can be used, in the docstring examples.
* Use different anime in the ``comparison`` method's key:value example
(in the docstring), so a wider range of ratings can be shown.

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* Rewrite ``calcs.pearson`` to make it 5x faster.
* Do the "standard deviation is zero" checking in ``calcs.pearson``,
so the standard deviation doesn't actually have to be calculated.
* Don't make ``MALRateLimitExceededError`` a descendant of
``MALAffinityException``.