DataFrame.to_json

[garawalid]

add to_json(). Part of #169

[codecov-io]

Codecov Report

Merging #238 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #238      +/-   ##
==========================================
+ Coverage   93.55%   93.57%   +0.02%     
==========================================
  Files          33       33              
  Lines        3272     3286      +14     
==========================================
+ Hits         3061     3075      +14     
  Misses        211      211

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
...tabricks/koalas/tests/test_dataframe_conversion.py	100% <100%> (ø)	⬆️
databricks/koalas/frame.py	94.81% <100%> (+0.04%)	⬆️

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[garawalid]

I skiped this test df.to_json(orient='table') because the output is different in Pandas 0.23.4 with python 3.5 and Pandas 0.24.2 with python 3.6.

Expected:
    '{"schema": {"fields":[{"name":"index","type":"string"},{"name":"col 1","type":"string"},{"name":"col 2","type":"string"}],"primaryKey":["index"],"pandas_version":"0.20.0"}, "data": [{"index":"row 1","col 1":"a","col 2":"b"},{"index":"row 2","col 1":"c","col 2":"d"}]}'
Got:
    '{"schema": {"fields":[{"name":"index","type":"string"},{"name":"col 1","type":"string"},{"name":"col 2","type":"string"}],"pandas_version":"0.20.0","primaryKey":["index"]}, "data": [{"index":"row 1","col 1":"a","col 2":"b"},{"index":"row 2","col 1":"c","col 2":"d"}]}'

[HyukjinKwon]

Looks good otherwise

[HyukjinKwon]

oops, can you rebase this, @garawalid ? looks your csv PR merge caused a conflict here :).

[garawalid]

@HyukjinKwon done!