- moved property._is_self_referential() to be more generalized; returns True for...

- moved property._is_self_referential() to be more generalized; returns True for any mapper.isa() relationship between parent and child, and indicates that aliasing should be used for any join/correlation across the relation.  allows joins/any()/has() to work with inherited mappers referencing the parent etc.
- the original _is_self_referential() is now _refers_to_parent_table() and is only used during "direction" calculation to indicate the relation is from a single table to itself