ENH/API: Accept DataFrame for isin
API: Series should respect index labels. This would be incompatible, but there hasn't been any official release since `isin` was merged.

Will close #4421.  Still a WIP for now. Needs more testing.

Thoughts on adding an argument to ignore the index? i.e.

``` python
In [150]: df = pd.DataFrame({'A': [1, 2, 3, 4], 'B': [2, np.nan, 4, 4]}, index=['a','b','c','d'])

In [151]: s = pd.Series([1, 3, 11, 12], index=['a','b','c','d'])

In [152]: df.isin(s)
Out[152]: 
       A      B
a   True  False
b  False  False
c  False  False
d  False  False

In [152]: df.isin(s, ignore_index=True)
Out[152]: 
       A      B
a   True  False
b  False  False
c   True  False
d  False  False
```

I would say there's no need for an `ignore_index` since you could just do `df.isin(s.values)`.
