Add Series.to_json and Series.to_csv

[HyukjinKwon]

No description provided.

[codecov-io]

Codecov Report

Merging #358 into master will decrease coverage by 0.03%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
- Coverage    94.4%   94.36%   -0.04%     
==========================================
  Files          36       36              
  Lines        3646     3656      +10     
==========================================
+ Hits         3442     3450       +8     
- Misses        204      206       +2

Impacted Files	Coverage Δ
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/frame.py	95.39% <ø> (-0.07%)	⬇️
databricks/koalas/generic.py	94.17% <90%> (-1.01%)	⬇️

---

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 3cffb28...8c70d0e. Read the comment docs.

[HyukjinKwon]

All tests apssed

[garawalid]

Nice. Thanks @HyukjinKwon

[HyukjinKwon]

Thanks for review @garawalid