Fixes #69 In many to many associations one side has to be the owner and
one side has to be the inverse end (it will be ignored when updating the
relationship values in the association table). References:

http://docs.jboss.org/hibernate/annotations/3.5/reference/en/html/entity.html
http://stackoverflow.com/questions/12552714/many-to-many-hibernate-inverse-side-ignored