Resampling with custom freq length: "Values falls before first bin"
```
print pandas.__version__
```

0.8.1

```
dtrange = pandas.date_range(start='1-1-2009', periods=21, freq='D')

raw = pandas.Series(np.arange(len(dtrange)), index=dtrange)
test1 = raw.resample('7D', how='count', closed='right')

print test1
```

2009-01-01    1
2009-01-08    7
2009-01-15    7
2009-01-22    6

```
test2 = raw.resample('7D', how='count', closed='left', label='left')
print test2
```

ValueError: Values falls before first bin

When trying to resample 21 days to three weeks i ran into the issue above. Closing to the right puts the 21 days in four bins, closing to the left (which i think does what i want) fails with the error above.

Changing the frequency from '7D' to 'W' makes the code run fine so it might have something to do with the 'custom' frequency length.
