Support persistence of "attributed relation" class as a Relationship

[andyjefferson]

If we have an M-N relation and we want to store some properties on the relation we create an intermediate model class, with the source and target objects present as well as the properties we want to store. Currently this will be stored as a Node, with Relationship to source Node, and Relationship to target Node. Would be a nice alternative to be able to persist objects of this type as a Relationship between Node for source and Node for target.

Would only apply to POJO that has 2 relation fields (source and target, and no other), and these objects cannot be null. The majority of SVN trunk already uses PropertyContainer instead of Node, and Neo4jUtils has a convenience method to detect if a class is marked as "attributed relation".