get_primary_key() works with inheritance and multpiple pks

* `contrib.sqla.tools.get_primary_key()` now works with inhteritance *and* multiple primary keys.

Every primary key is checked if it is a inherited one, and if it's inherited, the *current* column.key is returned. I *think* it should be safe just to use *prop.expression* here, but I am not absolutely sure.

( see http://docs.sqlalchemy.org/en/rel_0_8/orm/internals.html?highlight=columnproperty#sqlalchemy.orm.properties.ColumnProperty.expression )