Pivot table is making floats
I am having issues with pivot table creation making columns into floats when they don't need to be.

```
In [313]: f = pd.DataFrame({'a' : ['cat', 'bat', 'cat', 'bat'], 'v' : [1,2,3,4], 'i' : ['a','b','a','b']})

In [314]: z = pd.pivot_table(f, values='v', rows=['a'], cols=['i'], fill_value=0, aggfunc=np.sum)

In [315]: z
Out[315]: 
i    a  b
a
bat  0  6
cat  4  0

In [316]: z.b.dtype
Out[316]: dtype('float64')

In [317]: f.v.dtype
Out[317]: dtype('int64')

In [318]: np.sum(f.v)
Out[318]: 10

In [319]: type(np.sum(f.v))
Out[319]: numpy.int64
```

This is making stuff ugly when I to_csv it.
