Implement stack() for DataFrame.

[ueshin]

Adding stack() for DataFrame.

• https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.stack.html

Resolves #877.

[codecov-io]

Codecov Report

Merging #1329 into master will decrease coverage by 2.22%.
The diff coverage is 97.22%.

@@            Coverage Diff             @@
##           master    #1329      +/-   ##
==========================================
- Coverage   95.23%   93.01%   -2.23%     
==========================================
  Files          34       34              
  Lines        7373     7440      +67     
==========================================
- Hits         7022     6920     -102     
- Misses        351      520     +169

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	81.7% <ø> (-18.3%)	⬇️
databricks/koalas/frame.py	96.04% <97.22%> (-0.68%)	⬇️
databricks/koalas/usage_logging/__init__.py	26.12% <0%> (-69.37%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/missing/series.py	71.42% <0%> (-28.58%)	⬇️
databricks/koalas/missing/indexes.py	88.8% <0%> (-11.2%)	⬇️
databricks/koalas/__init__.py	91.48% <0%> (-2.13%)	⬇️
databricks/conftest.py	94.33% <0%> (-1.89%)	⬇️
... and 5 more

---

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 a849249...ec3ff49. Read the comment docs.

[HyukjinKwon]

LGTM otherwise.