Adds introspection rules for South

[South](https://pypi.python.org/pypi/South) reports an error when creating schema migrations using the richenum fields.

    ! Cannot freeze field 'myapp.mymodel.index_enum_field'
    ! (this field has class django_richenum.models.fields.IndexEnumField)
    ! Cannot freeze field 'myapp.mymodel.lax_index_enum_field'
    ! (this field has class django_richenum.models.fields.LaxIndexEnumField)
    ! Cannot freeze field 'myapp.mymodel.canonical_enum_field'
    ! (this field has class django_richenum.models.fields.CanonicalNameEnumField)
   
    ! South cannot introspect some fields; this is probably because they are custom
    ! fields. If they worked in 0.6 or below, this is because we have removed the
    ! models parser (it often broke things).
    ! To fix this, read http://south.aeracode.org/wiki/MyFieldsDontWork

Adding the [introspection rules](http://south.readthedocs.org/en/latest/customfields.html#extending-introspection) allows South to generate the schema.

The ruleset is conditionally blocked off for those not using South.