rolling_mean returns small negative value on Series with non-negative values
We came upon this issue today while trying to take a square root of a non-negative series. 

A quick search came up with this old issue: https://github.com/pydata/pandas/issues/2527.The issue was reproduced on 0.15 by tweaking the code from the previous report, which is below:

```
from pandas import date_range, Series
s = Series(index=date_range('1999-02-03','1999-04-05'))
s['1999-02-03'] = 0.00012456
s['1999-02-04'] = 0.0003
s['1999-04-04'] = -0.0
s['1999-04-05'] = -0.0
In [21]: s.rolling(1).mean()['1999-04-05']
Out[21]: -5.421010862427522e-20
```

which returns -5.4210108624275222e-20.
