Add Series.dtypes

[HyukjinKwon]

This is an alias of Series.dtype.

[codecov-io]

Codecov Report

Merging #355 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #355      +/-   ##
=========================================
+ Coverage   94.33%   94.4%   +0.07%     
=========================================
  Files          35      36       +1     
  Lines        3564    3647      +83     
=========================================
+ Hits         3362    3443      +81     
- Misses        202     204       +2

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	93.1% <100%> (+0.04%)	⬆️
databricks/koalas/__init__.py	92.59% <0%> (ø)	⬆️
databricks/koalas/tests/test_dataframe.py	100% <0%> (ø)	⬆️
databricks/koalas/tests/test_namespace.py	100% <0%> (ø)
databricks/koalas/namespace.py	92.95% <0%> (+1.92%)	⬆️

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[ueshin]

LGTM.

[ueshin]

Thanks! merging to master.