Inconsistent results using custom function though resample & how
I have noticed some inconsistent results between using resample & how calling a defined function.  It might be I have misunderstood how this is meant to function.

Here's some code to illustrate it:

```
import numpy as np
import pandas
from datetime import *

start = datetime(2010,6,1)
end = datetime(2012,1,1)

rng = pandas.date_range(start, end, freq='BM')

ts = pandas.Series(np.random.randn(len(rng)), index=rng)

def testfunction1(data):
    return np.sum(data) / len(data)

def testfunction2(data):
    return np.mean(data) / np.std(data)

t1 = ts.resample('Q', how=testfunction1) 

t0 = ts.resample('Q', how=np.mean)

t2 = ts.resample('Q', how=np.sum)

t3 = ts.resample('Q', how=len)

# Compare defined function with numpy function

t1 == t0

# Everything is True

# Compare defined function (t1) with explicit calculation on timeseries

t2/t3 == t1

# Everything is True

# Apply second function involving mean/std

d0 = ts.resample('Q', how=testfunction2)

d1 = ts.resample('Q', how=np.mean)

d2 = ts.resample('Q', how=np.std)

# Compare defined function with explicit calculation on timeseries

d1/d2 == d0

# Everything is False
```
