support subtraction of objects with as_seconds, analogous to addition

Also remove redundant tests from datetime.t.
test subtraction order for DateTime::Moonpig minus scalar object

a scalar object is one what represents an interval and which has an
->as_seconds method Such an object can be the right-side operand of
the - operator, but not the left-side operand.
default to UTC dates
rename ->iso to ->st since it is not actually ISO 8601 format
man page