Export post_date_gmt to YAML and add timezone info

I've changed from exporting local time to UTC and adding the timezone info into YAML. I'm trying to migrate my blog to awestruct and the YAML engine assumes (correctly according to spec) that timestamps without a timezone are not local time, but UTC, so the export in the original version was wrong for my use case.

By exporting post_date_gmt and adding the timezone information it should be correct for everyone who has wordpress set up with the correct timezone info.