Implement DataFrame.aggregate and agg

[HyukjinKwon]

No description provided.

[HyukjinKwon]

Will take a look more tomorrow .. I opened this to don't forget.

[codecov-io]

Codecov Report

Merging #796 into master will increase coverage by 0.01%.
The diff coverage is 93.33%.

@@            Coverage Diff            @@
##           master    #796      +/-   ##
=========================================
+ Coverage   94.28%   94.3%   +0.01%     
=========================================
  Files          32      32              
  Lines        5776    5828      +52     
=========================================
+ Hits         5446    5496      +50     
- Misses        330     332       +2

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/groupby.py	89.92% <100%> (+0.07%)	⬆️
databricks/koalas/frame.py	96.78% <88.23%> (-0.11%)	⬇️
databricks/koalas/config.py	87.35% <0%> (+8.92%)	⬆️

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[HyukjinKwon]

I am merging this to prepare the release.