added a third "mapper" to a many-to-many relationship that becomes the dependency...

added a third "mapper" to a many-to-many relationship that becomes the dependency in the "middle", thus allowing circular many-to-many relationships
added testcase to the 'double' test suite (whose name will change...)
small fix to table.get_col_by_original
added **kwargs to EagerLazyOption so other property options can be sent through