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.
BTW, regardless of whether we go with a database-table or config-file approach for the age group stuff, it was a really great/cool/awesome/brilliant idea to keep ``age_group`` around and just have it automatically be set based on ``birthdate``.