Unify environment marker support

Originally reported by: jaraco (Bitbucket: jaraco, GitHub: jaraco)

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In #14, we determined that setuptools currently has two suboptimal implementations of environment marker support. Let's unify that support and put together a single, official implementation that works on all supported platforms.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/122

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Merge pull request #164. Fixes #122.

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Merge pull request series from Steve Kowalik for updated and unified marker implementation. Fixes #122.