Standardize code style using Black

[HyukjinKwon]

This PR proposes to add Black into Koalas.

• dev/linter-python detects if codes are formatted or not by Black
• dev/reformat script that reformats the codes by using black.
• It still keeps 100 line length style, and ignores E231 and E203 at pycodestyle. See also Provide lists of lint / pycodestyle rules that Black makes obsolete psf/black#429 and Black incorrectly preserves trailing comma on one-line collection literals and function declarations psf/black#1202. Black automatically formats and violates several rules.
• Update contribution guide.

Resolve #755

[codecov-io]

Codecov Report

Merging #1301 into master will decrease coverage by 0.02%.
The diff coverage is 93.94%.

@@            Coverage Diff             @@
##           master    #1301      +/-   ##
==========================================
- Coverage   95.06%   95.04%   -0.03%     
==========================================
  Files          34       34              
  Lines        7233     7238       +5     
==========================================
+ Hits         6876     6879       +3     
- Misses        357      359       +2

Impacted Files	Coverage Δ
databricks/koalas/missing/common.py	100% <ø> (ø)	⬆️
databricks/koalas/frame.py	96.51% <ø> (+0.05%)	⬆️
databricks/conftest.py	96.22% <100%> (+1.88%)	⬆️
databricks/koalas/numpy_compat.py	90.9% <100%> (ø)	⬆️
databricks/koalas/missing/frame.py	100% <100%> (ø)	⬆️
databricks/koalas/usage_logging/usage_logger.py	100% <100%> (ø)	⬆️
databricks/koalas/missing/__init__.py	100% <100%> (ø)	⬆️
databricks/koalas/mlflow.py	94.87% <100%> (ø)	⬆️
databricks/koalas/missing/groupby.py	100% <100%> (ø)	⬆️
databricks/koalas/usage_logging/__init__.py	98.19% <100%> (ø)	⬆️
... and 26 more

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[HyukjinKwon]

cc @ueshin and @itholic

[ueshin]

LGTM.

[HyukjinKwon]

I manually pushed to keep the co-author - 30b3334

Let's see if it's really useful ...

[itholic]

nice!