Gender should be a enum selection from consts

[madmatt]

ID3 expects the gender field on PersonalDetails to be one of 'Male', 'Female', 'Unknown' or 'Unspecified' (alternatively it can be omitted when querying GB data).

• Class documentation

We should introduce some public const string GENDER_* consts for this, noting that in the past we've had issues querying the API with 'male' vs 'Male' (capitalisation used to matter as I recall, it may not anymore).