Enable exact birthdates

Adds support for collecting the exact birthdate from attendees. Fixes #586.

The database on live/staging servers will need the "birthdate" column added to the Attendees table and the AgeGroups table added before this code can be used on them.
Two things here:

1. I don't think this was supposed to be attached to the ``State`` class.  It refers to ``self.min_age`` and ``self.max_age``, which are columns on the ``AgeGroup`` class, so I think this was intended to be a method there.

2. Since this is a method which takes no arguments and simply returns a value based on the state of its instance, then it would be more idiomatic to make it a property, maybe called ``age_range`` (since it's returning a start and end, so calling it ``age_group_date`` is kind of confusing).