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.
quick note: when we deploy this change, I definitely want to be around to make sure everything goes smooth with the database.  we should definitely backup the DB as this is the first major column change we've made to uber since we started deploying with puppet and sqlalchemy.