Add ability to parse iso8601 durations.

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';
interval
----------
P7D

In order to better operate with those libraries, this library no longer
includes the week in the iso8601 format.
Merge pull request #13 from joevandyk/iso8601

Add ability to parse iso8601 durations.