Relationships on init

Allows for the creation of relationships via .new with properties and then a #save on the resulting object
Huh, yeah, that's a bit weird.  I must have not finished the spec.  It's supposed to be the ID of the Foo node to which a relationship should be created on save.  In that case, foo shouldn't be nil after save (unless I was trying to test here that it wouldn't make a relationship if the ID didn't exist...)