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.

[1]: https://docs.djangoproject.com/en/1.7/ref/models/fields/#field-choices
workshops/migrations/0027_auto_20141210_1546.py: Explicit person.gender choices

This is the template created with:

  $ python3.4 ./manage.py makemigrations
db.sql: Migrate to person.gender choices

With:

  $ make database
  $ python3.4 ./manage.py migrate
  $ sqlite3 db.sqlite3 .dump > db.sql