API: non-unique indexing on Series
The following currently are exceptions in master(the corresponding operations on DataFrames work though)

```
In [4]: s = Series([1,2,3,4],['foo','bar','foo','bah'])

In [5]: s.ix[['foo','bar','bah','bam']]
NotImplementedError: cannot handle non-unique indexing for non-DataFrame (yet)

In [6]: s[['foo','bar','bah','bam']]
AssertionError: Index length did not match values
```
