Add ILocIndexer.

[ueshin]

This PR adds loc property for DataFrame and Series.
The limitation is that the row access is only allowed to .iloc[:10, ...] (limit(10)) or .iloc[:, ...] (get all rows).
Resolves #247.

[codecov-io]

Codecov Report

Merging #364 into master will increase coverage by 0.06%.
The diff coverage is 92.18%.

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   94.36%   94.43%   +0.06%     
==========================================
  Files          36       36              
  Lines        3657     3754      +97     
==========================================
+ Hits         3451     3545      +94     
- Misses        206      209       +3

Impacted Files	Coverage Δ
databricks/koalas/missing/frame.py	100% <ø> (ø)	⬆️
databricks/koalas/missing/series.py	100% <ø> (ø)	⬆️
databricks/koalas/generic.py	94.33% <100%> (+0.16%)	⬆️
databricks/koalas/tests/test_indexing.py	87.5% <88.13%> (+2.24%)	⬆️
databricks/koalas/indexing.py	92.69% <95.38%> (+1.78%)	⬆️
databricks/koalas/__init__.py	88.88% <0%> (-3.71%)	⬇️

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[HyukjinKwon]

+1 ! sorry @ueshin, seems I caused the conflict. Let's get this in first before my other PRs.

[ueshin]

@HyukjinKwon no problem! that happens!