Some further cleanups in date/time/date-time formatting, and additional work towards user-defined formats
TemplateDateFormat-s work with TemplateDateModel-s instead of with Date-s. This is expected to be required for application-domain-aware formatting, where the TemplateModel-s may hold the application-domain meaning of the date (like it's not just a date-time, but the time of departure).
Added new date_format/time_format/datetime_format-s: iso, iso_z, iso_nz. This also means that now we can parse such dates. Minor improvements in date/time related code and related documentation.