Support 2-digit HH timezones as valid ISO 8601 (fixes #1357)

This updates the iso 8601 regex slightly to allow 2 digit timezones, [as specified in ISO 8601](http://www.cl.cam.ac.uk/~mgk25/iso-time.html#zone), to allow for inputs like: `2013-10-17 14:55:23-04`.

I've also taken the liberty of dropping the 'preliminary' from the comment above, since it's now been in since 1.6.0, coming up on 2 years ago.