Add missing method rename for koalas dataframe

[WeichenXu123]

Add missing method rename for koalas dataframe.

Some limitation:

• Do not support in-place operation.
• Require the mapper function include return type hint. such as:

def f1(x) -> int:
    return x*10

• When rename index labels, it is possible to raise SparkException instead of KeyError (Discussion: Could we get the nested exception "KeyError" and re-throw it ?)

[WeichenXu123]

One thing we need to discuss is:
when we want to rename labels in index, we'd better to let user specify the output label type. Because in koalas, the underlying spark dataframe transforming the index labels via pandas udf, which require the udf return type.
Currently, I force the return type to be the same with the old index label type.

[codecov-io]

Codecov Report

Merging #806 into master will decrease coverage by 0.24%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master     #806      +/-   ##
==========================================
- Coverage   94.29%   94.05%   -0.25%     
==========================================
  Files          32       32              
  Lines        5911     5988      +77     
==========================================
+ Hits         5574     5632      +58     
- Misses        337      356      +19

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/frame.py	95.95% <81.25%> (-0.87%)	⬇️
databricks/koalas/__init__.py	82.5% <0%> (-2.5%)	⬇️
databricks/conftest.py	95.34% <0%> (-2.33%)	⬇️
databricks/koalas/generic.py	94.94% <0%> (-0.51%)	⬇️

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[WeichenXu123]

@HyukjinKwon @ueshin Ready for review. I leave some discussions in PR description. When these discussion resolved, I will add some edge tests for them.

[HyukjinKwon]

I discussed about this offline and reviewed roughly @ueshin. Let me leave it to you or I review a day after tomorrow (after my vacation).

[ueshin]

@WeichenXu123 Thanks for working on this! I left some comments.

Btw, pandas supports to rename both index and columns at the same time.

>>> df
   A  B
0  1  4
1  2  5
2  3  6
>>> df.rename(index={0: 'x', 1: 'y', 2: 'z'}, columns={'A': 'a', 'B': 'b'})
   a  b
x  1  4
y  2  5
z  3  6

Do you have a plan to support this?

[WeichenXu123]

Updated code to support renaming index and columns at the same time. @ueshin
(although the level arguments will be shared by them, it looks a little weird)

[WeichenXu123]

Why the codecov failed ? the travis passed.

[HyukjinKwon]

It fails due to decreasing test coverage (see https://codecov.io/gh/databricks/koalas/compare/9186870f42749382451b14bf15c8723e5d68105d...e1839febd9cdfc896cca8b7f55e24fad8eb910ba/diff)

[ueshin]

I left some nits. Otherwise, LGTM.

[softagram-bot]

Softagram Impact Report for pull/806 (head commit: 3cd9394)

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[HyukjinKwon]

Thanks @WeichenXu123