- comparator_factory is accepted by all MapperProperty constructors. [ticket:1149]

- comparator_factory is accepted by all MapperProperty constructors. [ticket:1149]
- added other unit tests as per [ticket:1149]
- rewrote most of the "joined table inheritance" documentation section, removed badly out of
date "polymorphic_fetch" and "select_table" arguments.
- "select_table" raises a deprecation warning.  converted unit tests to not use it.
- removed all references to "ORDER BY table.oid" from mapping docs.
- renamed PropertyLoader to RelationProperty.  Old symbol remains.
- renamed ColumnProperty.ColumnComparator to ColumnProperty.Comparator.  Old symbol remains.