adding model adapter files in proper location with loading behavior

lib/cancan.rb

@@ -2,8 +2,10 @@ require 'cancan/ability'
 require 'cancan/rule'
 require 'cancan/controller_resource'
 require 'cancan/controller_additions'
-require 'cancan/active_record_additions'
-require 'cancan/mongoid_additions'
 require 'cancan/exceptions'
 require 'cancan/query'
 require 'cancan/inherited_resource'
+
+require 'cancan/model_adapters/active_record_adapter' if defined? ActiveRecord
+require 'cancan/model_adapters/data_mapper_adapter' if defined? DataMapper
+require 'cancan/model_adapters/mongoid_adapter' if defined? Mongoid

lib/cancan/model_adapters/data_mapper_adapter.rb

@@ -1,5 +1,3 @@
-require 'dm-core'
-
 module CanCan
   module Ability
     # could use alias_method_chain, but it's not worth adding activesupport as a gem dependency