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.
So here's how we generally handle dates:
* As you've seen, we typically call ``session.attendee(params, ...)`` to instantiate ``Attendee`` objects, and ``session.job(params)`` for ``Job`` objects, etc.  One of the things this utility method does is parse data based on column type, so like if you have an Integer column, it'll parse ``'3'`` to become ``3``.
* For ``UTCDateTime`` columns, we attempt to parse the datetime according to ``TIMESTAMP_FORMAT`` localized to ``EVENT_TIMEZONE`` (so we assume that all timestamps coming in from the client are using whatever timezone the event takes place in, Eastern Standard Time for MAGFest).  If it can't be parsed, we just don't set it and continue.  You can see this at https://github.com/magfest/ubersystem/blob/master/uber/models.py#L167
* So what this could let you do is make sure that your jQuery datepicker uses the same format, at which point you'd already have a ``datetime`` object by the time you got to these validation checks, or you'd have ``None`` which would imply that they didn't enter a birthdate.
* This means that you can still write validations here using that datetime without having to worry about parsing or mutating the values on the ``Attendee`` object you're checking.

In other words, you had the right idea about writing validations here; you'll still definitely want to do that.  You just need to split off the validation code (which will stay here) from the code which sets the ``age_group`` field (which will go in ``presave_adjustments``).

I hope that makes sense, let me know if any of that was unclear and I can elaborate further.  (Sorry I've been somewhat out of touch BTW, a combination of recovering from a really bad cold and being in crunch time at work has sapped away most of my MAGFest time and energy for the past few weeks.)