Add basic describe() function.

[ueshin]

This PR adds DataFrame.describe() and Series.describe() with basic stats for numeric types.
Resolves #334.

[HyukjinKwon]

LGTM if the tests pass. Let's get this in right away if the tests pass :D.

[codecov-io]

Codecov Report

Merging #375 into master will increase coverage by <.01%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #375      +/-   ##
==========================================
+ Coverage    94.6%   94.61%   +<.01%     
==========================================
  Files          41       41              
  Lines        4359     4383      +24     
==========================================
+ Hits         4124     4147      +23     
- Misses        235      236       +1

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/series.py	92.96% <100%> (+0.08%)	⬆️
databricks/koalas/frame.py	95.34% <93.75%> (-0.05%)	⬇️
databricks/koalas/generic.py	94.16% <0%> (-0.05%)	⬇️
databricks/koalas/datetime.py
databricks/koalas/datetimes.py	94.02% <0%> (ø)
databricks/koalas/tests/test_series_datetime.py	92.06% <0%> (+0.26%)	⬆️

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