Detect duplicate keys in dictionary literals

[pyflakes-bot]

Original report by icordasc (@sigmavirus24?) on Launchpad:

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It would be great if PyFlakes could catch users specifying duplicate keys in dictionary literals, e.g.,

d = {
    'a': 1,
    'b': 2,
    'a': 3,
}

That second 'a' would be useful to be warned about.

[jayvdb]

This was added in #72