Series.sum has inconsistent return type
Series.sum returns a numpy type, except when it's empty, in which case it returns a python int of value "0":

```
In [2]: type(pd.Series([0]).sum())
Out[2]: numpy.int64

In [3]: type(pd.Series().sum())
Out[3]: int
```

This poses a problem when I do `1 / myserie.sum()` because I expect to obtain `np.inf` rather than a divison by 0 exception.

I think the return type of `Series.sum()` for empty series should be inferred from the Series's dtype This way, `Series([], dtype='str').sum()` would return an empty string, and `Series([]).sum()` would return `np.float64(0)` since an empty series' default type seems to be float64.

Tested with Pandas 0.16.0
