Add pow, rpow, mod, rmod, floordiv, rfloordiv in DataFrame and Series

[HyukjinKwon]

This PR adds pow, rpow, mod, rmod, floordiv, rfloordiv in DataFrame and Series

[HyukjinKwon]

This finishes all operator-like implementations.

[codecov-io]

Codecov Report

Merging #503 into master will decrease coverage by 0.22%.
The diff coverage is 78.18%.

@@            Coverage Diff             @@
##           master     #503      +/-   ##
==========================================
- Coverage   93.87%   93.64%   -0.23%     
==========================================
  Files          30       30              
  Lines        4034     4077      +43     
==========================================
+ Hits         3787     3818      +31     
- Misses        247      259      +12

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/frame.py	94.41% <73.33%> (-0.8%)	⬇️
databricks/koalas/base.py	91.33% <75%> (-0.54%)	⬇️
databricks/koalas/series.py	93.23% <85.71%> (-0.42%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a96c01f...ea1fbe4. Read the comment docs.

[softagram-bot]

Softagram Impact Report for pull/503 (head commit: ea1fbe4)

⭐ Change Overview

(Open in Softagram Desktop for full details)

📄 Full report

• Permalink: Full report for pull/503

Give feedback on this report to support@softagram.com

[HyukjinKwon]

tests passed

[ueshin]

LGTM.

[ueshin]

Thanks! merging.

[HyukjinKwon]

Thanks @ueshin !!