Inline Series.unique to Pandas'

[HyukjinKwon]

This PR targets to inline Series.unique to Pandas'. After this PR:
Pandas

>>> import pandas as pd
>>> pd.DataFrame({"Person": ['a', 'b', 'b', 'c']})
  Person
0      a
1      b
2      b
3      c
>>> df = pd.DataFrame({"Person": ['a', 'b', 'b', 'c']})
>>> df.unique()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/pandas/core/generic.py", line 4376, in __getattr__
    return object.__getattribute__(self, name)
AttributeError: 'DataFrame' object has no attribute 'unique'
>>> df.Person.unique()
array(['a', 'b', 'c'], dtype=object)

Koalas

>>> import databricks.koalas as ks
>>> ks.DataFrame({"Person": ['a', 'b', 'b', 'c']})
  Person
0      a
1      b
2      b
3      c
>>> df = ks.DataFrame({"Person": ['a', 'b', 'b', 'c']})
>>> df.unique()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/hyukjin.kwon/workspace/forked/koalas/databricks/koalas/frame.py", line 1528, in __getattr__
    return Series(self._sdf.__getattr__(key), self, self._metadata.index_info)
  File "/Users/hyukjin.kwon/workspace/forked/spark/python/lib/pyspark.zip/pyspark/sql/dataframe.py", line 1296, in __getattr__
AttributeError: 'DataFrame' object has no attribute 'unique'
>>> df.Person.unique()
0    c
1    b
2    a
Name: Person, dtype: object

Resolves #233

[codecov-io]

Codecov Report

Merging #249 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #249      +/-   ##
==========================================
+ Coverage   92.17%   92.44%   +0.26%     
==========================================
  Files          35       35              
  Lines        3158     3203      +45     
==========================================
+ Hits         2911     2961      +50     
+ Misses        247      242       -5

Impacted Files	Coverage Δ
databricks/koalas/frame.py	92.87% <ø> (+0.92%)	⬆️
databricks/koalas/series.py	91.69% <100%> (+0.41%)	⬆️
databricks/koalas/tests/test_dataframe.py	100% <0%> (ø)	⬆️
databricks/koalas/tests/test_series.py	100% <0%> (ø)	⬆️
databricks/koalas/tests/test_utils.py	100% <0%> (ø)	⬆️
databricks/koalas/utils.py	100% <0%> (ø)	⬆️
databricks/koalas/namespace.py	90.34% <0%> (+0.41%)	⬆️

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[rxin]

nit: i don't think you need this since examples is literally one line below. i'm going to merge this and remove this line.