Also parse weeks and days in video’s duration

YouTube API does not fully respect ISO 8601 duration standard when
returning the "duration" of a video. This commit adds support for
videos with "days" and "weeks" in their duration which, although not
part of ISO 8601, are sometimes returned by YouTube API.
Merge pull request #56 from Fullscreen/add-days-and-weeks-to-duration

Also parse weeks and days in video’s duration