Trunk 4446: Support deriving auto_expire_date from duration and duration units for Drug Orders

Changes for ticket https://issues.openmrs.org/browse/TRUNK-4446
Its ok, date/time formats are hard to agree upon :) 

`yyyy-MM-dd` is ISO8601 date format http://www.iso.org/iso/home/standards/iso8601.htm. `Context.getDateFormat()` depends on locale, we can not use it.