BUG: Grouper(key='A') gives AttributeError when applying function
I was playing a bit with `pd.Grouper`. Shouldn't the following work:

```
In [109]: df = pd.DataFrame({'A':[0,0,1,1,2,2], 'B':[1,2,3,4,5,6]})

In [110]: df
Out[110]:
   A  B
0  0  1
1  0  2
2  1  3
3  1  4
4  2  5
5  2  6

In [111]: df.groupby(pd.Grouper(key='A')).sum()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-111-b9e051aabea3> in <module>()
----> 1 df.groupby(pd.Grouper(key='A')).sum()

C:\Anaconda\lib\site-packages\pandas\core\groupby.pyc in f(self)
    108             raise SpecificationError(str(e))
    109         except Exception:
--> 110             result = self.aggregate(lambda x: npfunc(x, axis=self.axis))

    111             if _convert:
    112                 result = result.convert_objects()

C:\Anaconda\lib\site-packages\pandas\core\groupby.pyc in aggregate(self, arg, *a
rgs, **kwargs)
   2634                 return getattr(self, cyfunc)()
   2635
-> 2636             if self.grouper.nkeys > 1:
   2637                 return self._python_agg_general(arg, *args, **kwargs)
   2638             else:

AttributeError: 'Int64Index' object has no attribute 'nkeys'
```
