Accept timezone-offsets in parseISO8601

I've done manual testing with various formats:
2014-03-28T10:26:00-0700
2014-03-28T10:26:00+0700
2014-03-28T10:26:00-07:00
2014-03-28T10:26:00+07:00
2014-03-28T10:26:00Z

Fixes #1