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), 'O' (for other), and NULL [1] (we don't know)
in the database.  Telling Django about these choices [2] gets us the
convenient get_FOO_display field and better form UI (drop down
selections) and validation.  The field also sets 'blank=True' [3] to
allow folks to edit their profile without specifying any of the gender
choices.

[1]: https://docs.djangoproject.com/en/1.7/ref/models/fields/#null
[2]: https://docs.djangoproject.com/en/1.7/ref/models/fields/#field-choices
[3]: https://docs.djangoproject.com/en/1.7/ref/models/fields/#blank
workshops/migrations/0027_auto_20141222_0144.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
Merging @wking's updates to gender selection and re-doing changes to migrater.

Instead of `------`, the pulldown for choosing gender should display `unknown`.