ENH: Add barh plot for DataFrame

[charlesdong1991]

kdf and pdf with specified x and y:

kdf and pdf without specified x and y:

[codecov-io]

Codecov Report

Merging #698 into master will decrease coverage by 0.05%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #698      +/-   ##
==========================================
- Coverage   93.71%   93.65%   -0.06%     
==========================================
  Files          32       32              
  Lines        5523     5534      +11     
==========================================
+ Hits         5176     5183       +7     
- Misses        347      351       +4

Impacted Files	Coverage Δ
databricks/koalas/plot.py	94.73% <92.85%> (-0.13%)	⬇️
databricks/koalas/__init__.py	82.05% <0%> (-2.57%)	⬇️
databricks/conftest.py	95.34% <0%> (-2.33%)	⬇️
databricks/koalas/frame.py	94.75% <0%> (-0.08%)	⬇️

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[charlesdong1991]

cannot figure out how to resolve codecov issue. 😥

if you could take a look at either barh or bar, once either of them gets merged, I could fix duplicates/conflict for the other one, since there are duplicate codes shared in these two PRs.

[HyukjinKwon]

It's okay. codecov sometimes looks a bit dumb.

[softagram-bot]

Softagram Impact Report for pull/698 (head commit: 4e8d32e)

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[charlesdong1991]

Conflicts are resolved. Let's see if everything can pass. @HyukjinKwon

I think I also need to resolve conflict for another PR once this is merged, so let me do it one by one.

[HyukjinKwon]

Thanks, merged to master.