Bug in pd.Series.mean()
In some cases, the mean is computed incorrectly.  Numpy however does the correct calculation.  There is no problem with the standard deviation calculation.  The following is an example.

```
In [11]: np.array(stateemp.area.tolist()).mean()
Out[11]: 23785.447812211703

In [12]: stateemp.area.mean()
Out[12]: 58.927762478114879

In [13]: np.array(stateemp.area.tolist()).std()
Out[13]: 22883.862745218048

In [14]: stateemp.area.std()
Out[14]: 22883.864924811925

In [15]: pd.__version__
Out[15]: '0.13.1'

In [16]: np.__version__
Out[16]: '1.8.1'
```
