New macros for many-to-many relationships

Hello Chris, i enjoyed using your great korma library but i missed the many-to-many relations feature so i implemented 2 new macros: has-many-to-many and belongs-to-many-to-many. Maybe their naming is little bit crude but i didn't want it to be confusing with existing 'has-many' and 'belongs-to' macros creating many-to-one and one-to-many relations. When 'with' macro is used, it works the same way as with 'has-many' relation, so referenced sub-entity is lazily fetched with post select for each main record from result-set. 
Of course mapping table entity2entity must exist for many to many relationships, default name for this table is entity2sub-entity when using:

(defentity entity
  (has-many-to-many sub-entity))

and sub-entity2entity when using:

(defentity entity
  (belongs-to-many-to-many sub-entity))

this default name could be overriden in supplied opts map (key :map-table), as is it possible with keys in mapping table.