Handle the 'Z' time zone designator in ISO 8601 time stamps

Sending a task message with an eta value using the Z time zone designator currently results in an error such as:

InvalidTaskError: invalid eta value '2014-02-09T02:17:06.007Z': tzinfo argument must be None or of a tzinfo subclass, not type 'str'
Better error when missing argument to preparse option.  Closes #1860