Implement DataFrame.unstack() but without level

[HyukjinKwon]

This PR implements DataFrame.unstack(). This PR intentionally does not implement level with multi-index. See the discussion at #886
When multi-index is not use, it is okay to implement.

Resolves #886

[codecov-io]

Codecov Report

Merging #1295 into master will decrease coverage by 0.03%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master    #1295      +/-   ##
==========================================
- Coverage   95.15%   95.11%   -0.04%     
==========================================
  Files          34       34              
  Lines        7238     7292      +54     
==========================================
+ Hits         6887     6936      +49     
- Misses        351      356       +5

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/frame.py	96.5% <95.23%> (-0.02%)	⬇️
databricks/koalas/__init__.py	85.1% <0%> (-8.52%)	⬇️
databricks/koalas/usage_logging/__init__.py	97.29% <0%> (-0.91%)	⬇️
databricks/koalas/testing/utils.py	78.51% <0%> (-0.47%)	⬇️
databricks/koalas/window.py	96.31% <0%> (-0.12%)	⬇️
databricks/koalas/series.py	96.39% <0%> (-0.04%)	⬇️
databricks/koalas/internal.py	96.07% <0%> (-0.02%)	⬇️
databricks/koalas/base.py	97.29% <0%> (ø)	⬆️
databricks/conftest.py	96.22% <0%> (ø)	⬆️
... and 25 more

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[ueshin]

LGTM.