Fix for audit_log ForeignKey on custom AUTH_USER_MODEL.

We were getting an error when using "audit_log = AuditLog()" on a custom user model (inheriting from django.contrib.auth.mdoels.AbstractBaseUser):
```
$ manage.py validate
CommandError: System check identified some issues:

ERRORS:
karaage.PersonAuditLogEntry.action_user: (fields.E300) Field defines a relation with model 'karaage.Person', which is either not installed, or is abstract.

System check identified 1 issue (2 silenced).
```
The quick and dirty fix was to compare the kwargs['related_name'] with the relation target (to) and use "self" instead.  We're open to alternate fixes...