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.
Good points - the original function I wrote actually took arguments, but in its current form it does belong elsewhere.

I'll be flipping around the logic on this function today, so it's actually going to end up in the Attendee class. (my thinking is, it's easier/faster to calculate the attendee's age once per transaction than to calculate birthdate ranges for several age groups)