add support for list of strings as default value for fields

For my accounts schema I want to have a default value for the roles field. This is the schema should look like this:

```
'roles': {
    'type': 'list',
    'allowed': ['user', 'superuser', 'admin'],
    'default': ['user']
}
```

This worked in the past, but behavior seems to have changed in version 0.4. Now the default value is simply ignored. I can restore the old behavior with these changes and I think there are no site effects.

@nicolaiarocci would be nice if you could have a look at this.

Regards