Defrost

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* sort packages on name and then version such that ``foo==2.0`` comes before
``foo-bar==1.0`` which is not the case when both are treated as plain
strings.

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* implement ``Requirement``
* implement ``Package``
* implement ``PipFreeze.__contains__()``
* implement ``PipFreeze.__len__()``
* drop py26 support
* remove ``PipFreeze.satisfies_requirement()``
* implement ``PipFreeze.load_requirements()``

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* implement ``PipFreeze.__bool__()`` (py3) and ``PipFreeze.__nonzero__()`` (py2)
* implement ``PipFreeze.__iter__()``

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* implement ``pipfreeze.PipFreeze``