Business day resampling 
Hi everyone, 

I just stumbled across this odd behaviour: 

For a daily time series, I would expect `resample('B',  how='last')` to be equivalent to selecting rows whose weekday is not Saturday or Sunday.  However, `resample` groups together Fridays,  Saturdays and Sundays and hence we get Sunday's value on Friday.

In the time series context, I guess most people would expect `resample('B', ...)` to bin Saturdays, Sundays and Mondays, so that **no looking into the future** occurs.

Am I missing something? 
