Changes to autodetect the datetime unit from the form of the ISO 8601 string. This makes np.datetime64('2011-03-12') have a 'D' unit, for example. Also provide automatic units for 'now' and 'today', and have them throw errors when used with the wrong kind of unit.
ENH: datetime: Automatically detect the unit for scalar construction
ENH: datetime: Make 'now' and 'today' only parse with appropriate units
In particular, 'now' needs time-like units, and 'today' needs
Moved to datetime_dev pull request.