Defrost

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* implement ``PipFreeze.__getitem__`` and ``PipFreeze.get()``.

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* gracefully handle blank lines and lines starting with dash
(https://github.com/SurveyMonkey/defrost/pull/14)

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* gracefully handle comments that may be present in pip freeze output

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* introduce ``defrost-lint`` command to test the validity of requirement files.

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* remove attributes ``Package.raw`` and ``Requirement.raw``, instead use
``Package.__str__()`` and ``Requirement.__str__()``.
* ignore links found in pip freeze output (-f or -e lines)
* Introduce the notion of deprecation severity. ``Package.deprecate()`` now
takes a severity kwarg which defaults to ``"error"`` and a requirement entry
in the YAML file now accepts an optional ``severity`` which can be set to
``error`` or ``warn``. This affects the exit status code for the command line
interface.
* ``PipFreeze.load_requirements()`` would choke if a reason was not provided

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* add attribute ``Package.deprecated_by``
* method ``Package.deprecate()`` takes an optional ``deprecated_by`` argument.
* add command line utility to list deprecated packages given a requirement file
and a pip freeze output file.
* rename project pipfreeze to defrost to avoid confusion with the command
``pip freeze``.