Add possibility to define multiple relations to same model,, fix many to many joins and relations

• Allow multiple relations to the same related model/table.
• Fix for wrong relation column used in many_to_many relation joins (fix #73)
• Fix for wrong relation population for m2m relations when also fk relation present for same model.
• Add check if user provide related_name if there are multiple relations to same table on one model.
• More eager cleaning of the dead weak proxy models.