Add ability to parse iso8601 durations.

[joevandyk]

I'm not an ISO8601 expert, but it appears many implementations ignore weeks in the format string. The Ruby iso8601 library does, and postgresql does as well:

select interval '1w';
P7D

In order to better operate with those libraries, ruby-duration no longer includes the week in the iso8601 format.

[peleteiro]

Thanks