Add GroupBy.shift.

[ueshin]

Resolves #900.

[codecov-io]

Codecov Report

Merging #910 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #910      +/-   ##
==========================================
+ Coverage   94.28%   94.33%   +0.04%     
==========================================
  Files          34       34              
  Lines        6265     6319      +54     
==========================================
+ Hits         5907     5961      +54     
  Misses        358      358

Impacted Files	Coverage Δ
databricks/koalas/missing/groupby.py	100% <ø> (ø)	⬆️
databricks/koalas/groupby.py	91.37% <100%> (+0.23%)	⬆️
databricks/koalas/missing/series.py	100% <0%> (ø)	⬆️
databricks/koalas/frame.py	95.98% <0%> (+0.04%)	⬆️
databricks/koalas/series.py	95.46% <0%> (+0.16%)	⬆️
databricks/koalas/testing/utils.py	78.39% <0%> (+0.4%)	⬆️

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[ueshin]

Thanks! merging.