Iso datetime format can be truncated.

Correct ISO8601_REGEX to support truncated time part in the
datetime representation, and add corresponding tests: now we support
time stamps truncated to the minute, the hour or the day:

    2007-01-25T12:00Z
    2007-01-25T12Z
    2007-01-25
Merge branch 'contributor_agreement' into iso8601_truncation