passing np.std to resample does not actually call the numpy function
```
Hi,
Again the std issue
When I call numpy.std outside pandas I get the population std where as if I call inside it gives me sample std. How can I ask pandas to allow np.std to behave as it originally does?
wit best regards,
Sudheer

In [13]: ts[53153:53155]
Out[13]:
2007-09-15 15:30:00 30.1
2007-09-15 15:40:00 31.6
Freq: 10T

In [14]: np.std(ts[53153:53155])
Out[14]: 0.75

In [15]: ts[53153:53155].resample('D',how=np.std)
Out[15]:
2007-09-15 1.06066
Freq: D
```
