Sum of grouped bool column has inconsistent type
Summing a bool column after a groupby gives a bool result until there are two or more True values, when it becomes a float64.  Seems like it should always be an (unsigned?) integer.  Straight sum without a groupby always gives an int64. This is with 0.13.1.

```
pd.DataFrame([True]).groupby(lambda x: 0).sum()
      0
0  True

pd.DataFrame([True,True]).groupby(lambda x: 0).sum()
   0
0  2

pd.DataFrame([False]).groupby(lambda x: 0).sum()
       0
0  False

pd.DataFrame([False,False]).groupby(lambda x: 0).sum()
       0
0  False

pd.DataFrame([False,False,True]).groupby(lambda x: 0).sum()
      0
0  True

pd.DataFrame([False,False,True,True]).groupby(lambda x: 0).sum()
   0
0  2

pd.DataFrame([False,False]).sum()
0    0
dtype: int64
```
