DateRange - minute vs. minutes
I tried to create a DateRange in five minute steps.

`In [40]: DateRange(datetime(2011,11,11), datetime(2011,11,12), offset=DateOffset(minute=5))`

The %CPU is 100  and it consumes a lot of memory but nothing happens, no result and no error. I continued to experiment and realized that i wrote 'minute' instead of 'minutes'

`In [41]: DateRange(datetime(2011,11,11), datetime(2011,11,12), offset=DateOffset(minutes=5))`

My suggestion is to maybe add some kind of argument check.
