Understand await / async

[pylint-bot]

Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: @PCManticore?)

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We should add inference support for the new coroutines added in Python 3.5. Targeted for astroid 1.5 mostly.

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• Bitbucket: https://bitbucket.org/logilab/astroid/issue/202