Working toward gender selection

1.  Merged branch 'gender-choices' of github.com:wking/amy into gender-choices
2.  Modified `migrater.py` to import data from legacy DB correctly (field order changes every time fields are added or modified).
3.  Modified `db.sql` based on these changes.

Problem:

1.  Edit a person: gender options are `---`, `Male`, `Female`, `Other`.
2.  Cannot change gender to `---`, but need to be able to (we don't know gender for most participants, and some other people prefer not to say).

What's the right way to handle "may be null" for an enumeration?  Make "Unknown" an explicit value (and the default)?

Note that this is based off `swcarpentry/master` - you may wish to merge that into your branch before looking at this one.