Implement durations, addition and subtraction

Test scripts for C<clone> and C<last_day_of_month>

Check test coverage for other cases (which cases?)

printf specifiers C<%V>, C<%w>

Maybe support the English language, at least for the C<month_name>
and the C<day_name> methods.

Install the other F<DateTime::Calendar> modules and use them in the F<today> example

