Implement ISO 8601 conversions

There's an ISO 8601 format not only for dates, but also for durations without date, and it's surprisingly close to what this module uses for internal representations of duration data. Also see the Wikipedia entry for a little introduction (which might even suffice for implementing this stuff).

We want conversion of duration arrays (...in fact, having a duration object would be even nicer) to duration strings as defined in the standard, and back. Also, we want to have "weeks" as a possible metric (mutually exclusive with "months"), which is a bit dangerous because it introduces ambiguousness in the data representation, but that will still be worth the effort.