Support for bind_as()

Please review.

You can use it like:

    m=LDAPUser.objects.get(username='m')
    m.password=['2345']
    m.bind_as(alias='ldap-user', password='1234').save()

or, more cleaner:

    m=LDAPUser.objects.get(username='m').bind_as(alias='ldap-user', password='1234')
    m.password=['2345']
    m.save()

or for admin:

    m=LDAPUser.objects.get(username='m').bind_as(alias='ldap-admin', dn='...', password='...')
    m.password=['2345']
    m.save()

*alias* is a free-form string that will be used in `settings.DATABASES`. It is important that it is unique for credentials since previous alias is reused without checking `dn` and `password`.

*dn* specifies DN to use. If unspecified, defaults to DN of current object.