workshops/models.py: List choices for Person.gender

This is not a free-form field, since we're only allowing 'M' (for
male), 'F' (for female), and 'O' (for other) in the database.  
[Telling Django about these choices][1] gets us the convenient
get_FOO_display field and better form UI (drop down selections) and
validation.

This is based on #92, so that should be merged first.  We need #92 to
handle the event.notes and site.notes migrations otherwise the
makemigrations we used for person.gender would pull those in as well.

[1]: https://docs.djangoproject.com/en/1.7/ref/models/fields/#field-choices