Use full timestamp so backups during the same minute/DST changes are unique

When testing playbooks on a single host, it's easily possible to make multiple changes to a file within a single minute, and it's possible for there to be confusion during daylight savings rollover.

This patch includes the time zone without resorting to dateutil.tz.

I tested by applying the patch to my installed copy of 1.7.2, as I lack all the dependencies for a full test.

Example output:
grub.conf.2014-11-20@21:22:46.393099-0800~
grub.conf.2014-11-21@06:36:32.014887+0100~