Record Accessors (#977)

Removing AR specific code from Resource and moving to new RecordAccessor class.

Author: lgebhardt

lib/jsonapi-resources.rb

@@ -24,3 +24,5 @@
 require 'jsonapi/operation_result'
 require 'jsonapi/callbacks'
 require 'jsonapi/link_builder'
+require 'jsonapi/record_accessor'
+require 'jsonapi/active_record_accessor'