API: implement non-unique indexing in series (GH4246)

closes #4246

```
In [3]: s = Series([1,2,3,4],['foo','bar','foo','bah'])

In [4]: s.ix[['foo','bar','bah','bam']]
Out[4]: 
foo     1
foo     3
bar     2
bah     4
bam   NaN
dtype: float64

In [5]: s[['foo','bar','bah','bam']]
Out[5]: 
foo     1
foo     3
bar     2
bah     4
bam   NaN
dtype: float64
```