Make PrettyTime play nice with Django F expressions

[jdotjdot]

Right now, though you can use the .then() method for anchoring on arbitrary dates or datetimes, it seems to throw an error when used with Django F expressions, like so:

expiry_date = models.DateField(default=lambda: t(3).months.from_.then(F('publish_date')))

For now, users will have to make do with the following, which does work:

expiry_date = models.DateField(default=lambda: F('publish_date') + relativedelta(months=+3))