Add ability for an ActiveRecord to automatically index associations.

This module adds the ability for an ActiveRecord to trigger Solr indexing of associated ActiveRecord models. The associated models are indexed after the base model is saved or destroyed.

Note that the trigging model does not need to be Searchable, but the associated model(s) do need to be Searchable.

Example

     class Post < ActiveRecord::Base
       belongs_to :blog
       belongs_to :author
     
       index_related :blog, :author, if: :published?
     end

We needed this feature because we had to reindex parent objects when the child objects changed.