API/ENH: str.split should return a DataFrame
I find the behavior of `str.split` a bit odd, and it should by default just return a `DataFrame` (or maybe have a new function / option). Its straightforward to coerce it, but could/should be done internally.

(and `str.extract` does return a DataFrame IIRC)

```
In [22]: s
Out[22]: 
0            apple
1    apple, orange
2           orange
dtype: object

In [23]: s.str.split(',\s+')
Out[23]: 
0            [apple]
1    [apple, orange]
2           [orange]
dtype: object

In [24]: s.str.split(',\s+').apply(Series)
Out[24]: 
        0       1
0   apple     NaN
1   apple  orange
2  orange     NaN
```
