Add koalas.concat

[HyukjinKwon]

This PR adds base of ks.concat. There are still many parameters to implement existent in Pandas'

Resolves #231

[codecov-io]

Codecov Report

Merging #348 into master will increase coverage by 0.07%.
The diff coverage is 98.34%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
+ Coverage   94.31%   94.38%   +0.07%     
==========================================
  Files          35       36       +1     
  Lines        3551     3633      +82     
==========================================
+ Hits         3349     3429      +80     
- Misses        202      204       +2

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	92.59% <ø> (ø)	⬆️
databricks/koalas/tests/test_namespace.py	100% <100%> (ø)
databricks/koalas/tests/test_dataframe.py	100% <100%> (ø)	⬆️
databricks/koalas/namespace.py	92.95% <97.1%> (+1.92%)	⬆️

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[ueshin]

LGTM.

[ueshin]

Thanks! merging to master.