Implemented Series.argsort

[itholic]

This PR proposes Series.argsort

>>> kser = ks.Series([3, 3, 4, 1, 6, 2, 3, 7, 8, 7, 10])
>>> kser
0      3
1      3
2      4
3      1
4      6
5      2
6      3
7      7
8      8
9      7
10    10
dtype: int64

>>> kser.argsort().sort_index()
0      3
1      5
2      0
3      1
4      6
5      2
6      4
7      7
8      9
9      8
10    10
dtype: int64

[ueshin]

The result in the description is different from pandas'.

>>> pd.Series([3, 3, 4, 1, 6, 2, 3, 7, 8, 7, 10]).argsort()
0      3
1      5
2      0
3      1
4      6
5      2
6      4
7      7
8      9
9      8
10    10
dtype: int64

[itholic]

The result in the description is different from pandas'.

>>> pd.Series([3, 3, 4, 1, 6, 2, 3, 7, 8, 7, 10]).argsort()
0      3
1      5
2      0
3      1
4      6
5      2
6      4
7      7
8      9
9      8
10    10
dtype: int64

Oh, sorry I should've updated PR description.

[itholic]

Thanks, @ueshin !

[itholic]

Anyway, I want to include @ueshin as an author for this work.

Maybe merged pull request in my repository is enough, or do I have to do something ??

[itholic]

Nvm about #1793 (comment) , think I'll be able to address it !

[ueshin]

LGTM.
Could you fix the code conflicts?

[itholic]

Oops. why the co-author didn't applied ? Sorry let me figure out the way.