Add automatic alias tracking to ActiveRelationResourceFinder

* enables related resource filtering
* introduces JoinTree to track the required joins for a request to avoid
join collisions
* simplifies the resource finder, but there's still some room for improvement
* makes use of the rails 5 `left_join` method. For rails 4 will require
an additional gem to add the functionality. This is tested with the
`left_join` gem, though others could exist and work.

Author: lgebhardt

Gemfile

@@ -19,5 +19,6 @@ when 'master'
 when 'default'
   gem 'railties', '>= 5.0'
 else
+  gem 'left_join' if version.start_with?('4.2')
   gem 'railties', "~> #{version}"
 end

lib/jsonapi-resources.rb

@@ -25,4 +25,5 @@
 require 'jsonapi/callbacks'
 require 'jsonapi/link_builder'
 require 'jsonapi/active_relation_resource_finder'
+require 'jsonapi/active_relation_resource_finder/join_tree'
 require 'jsonapi/resource_identity'