Add restrict_only to brainstem associations.

+ Fixes YARD documentation.  r on attr_accessors should be rw.
+ Adds ability to specify a :restrict_only option on AssociationFields that restrict them to only be used in "only" queries (defaults to false).

**Usage**:

    :files => association(:json_name => "files", :restrict_only => true) { |story| story.files }

It also needs the ability to preload associations as well but I think that is a separate PR.