date_range treats '.5s' as '5s'
I'm new to pandas, so maybe I'm missing something, but I'd expected freq='.5s' to give me 500ms intervals, not 5s intervals:

> > > pd.date_range(0, periods=5, freq='.5s')[1]
> > > Timestamp('1970-01-01 00:00:05', offset='5S')
> > > pd.date_range(0, periods=5, freq='500ms')[1]
> > > Timestamp('1970-01-01 00:00:00.500000', offset='500L')
> > > pd.**version**
> > > '0.14.1'
> > > import sys; sys.version
> > > '3.4.1 (default, Jul  6 2014, 20:01:46) \n[GCC 4.9.0]'

Behaviour is the same under Python 2 and 3.
