Background setup for birthdate

Adds the config option and the new age_groups table in models.py.

Also includes a first-pass function to get the birthdates that
correspond to age groups.
Adds exact birthdate forms

Makes public and admin forms check for collect_exact_birthdate and
display a different form if true.
Merge pull request #7 from Anthrocon-Reg/ac-staging

Update enable_birthdate with ac-staging
Rewrite get_age_group_date

Rewrite of the get_age_group_date to be based on the database min_age
and max_age.
Adds birthdate/age-group conversion and reg check

model_checks.py gets a big ugly function that takes a mm-dd-yy formatted
date and finds its associated age group.

The method for doing so is non-ideal - the age_group options must have
descriptions matching the database entries - but it will be usable
without any data loss.

This function also throws an error if the age group's can_register value
is False.
Front-end changes for birthdate

Adds the front-end input fields for birthdate, including correct
formatting.

Also adds the jQueryUI datepicker class.
Make birthdate unrestricted
Fixes age_group

Fixes a typo that caused age_group to have no default.
Revert "Adds support for anthrocon_plugin"

This reverts commit 4395b14f4d1201104a4e53a1bc82f158aa1e54a2.
Fix birthdate sessions and use relational database

Age group is no longer added to two sessions. It is now calculated by
attendee age, not the age groups' birthdate ranges. The database tables
were made relational and most references to specific age groups were
removed, though that part needs some work. Fixes #586.
Merge pull request #16 from Anthrocon-Reg/enable_birthdate

Enable birthdate