Yapf linting

[pcmoritz]

This is linting the Python codebase with yapf and automatically checking on travis if files have linting errors. Currently excludes rllib and dataframes.

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[concretevitamin]

LGTM feel free to merge if tests pass

[robertnishihara]

@richardliaw note that this currently does linting for Tune (but not RLlib or Pandas on Ray). Is now a reasonable time for tune?

[AmplabJenkins]

Test FAILed.
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Test FAILed.

[richardliaw]

Tune is fine right now; thanks for checking!

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[alok]

I'd tweak the style file to

1. split before the first argument on long function calls since it makes it easy to quickly scan the argument list (split_before_first_argument = True)
2. not join lines to make single line if statements (breaks visual flow since you usually expect the indent) (join_multiple_lines = False)
3. always split if comma terminated since the clean diffs created by trailing commas only works if there's one item per line (split_arguments_when_comma_terminated = True)

But those are just minor nitpicks. Glad to see this.