Yummly

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- Updated ``Meta*`` model classes based on changes to Yummly API. Thanks flyingfrog81_!

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- Fix indentation issue in models.py. Thanks fredthekid_!

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- Saner defaults for API data attributes
- Use ``**kargs`` in model class ``__init__``s to avoid duplication
of defaults/assignments
- Use ``kargs.get()`` for API data attributes that may or may not be
available to avoid key errors

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- Fetch recipe ``yield`` keyword attribute safely from response since
it's not guaranteed to be present
- Supply defaults for ``recipe.nutritionEstimates``, ``recipe.images``,
and ``recipe.yields``
- Ensure ``totalTime`` and ``totalTimeInSeconds`` are set to ``0`` when
falsey

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- Change default value for ``Recipe.totalTimeInSeconds`` to ``0``:
mirror default returned by search results
- Change default value for ``SearchMatch.sourceDisplayName`` to ``''``:
field not guaranteed to exist in return

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- Handle top-level keys that are not always present in Yummly
responses.
- Provide default values for ``Recipe`` fields: ``rating=0``,
``flavors={}``, ``totalTime=0``, ``totalTimeInSeconds=''``,
``numberOfServings=0``, ``yields=''``, ``attributes={}``.
- Assert that ``Client`` ``timeout`` parameter is >= 0.
- Assert that ``Client`` ``retries`` parameter is >= 0 and an integer.