ENH date_range should accept dateutil frequency constants
Currently date_range accepts string flags for `freq=`.

dateutil (which is already a pandas dependency) has constants for `YEARLY, MONTHLY, WEEKLY, DAILY, HOURLY, MINUTELY, SECONDLY` which are used in the dateutil equivalent of date_range, `rrule`.

it would be a nice feature if these were accepted as well as the string arguments
