Add after_commit support

If not, application crashes on Heroku.

lib/mongoid_paperclip.rb

@@ -62,6 +62,26 @@ def self.included(base)
     end
 
     module ClassMethods
+
+      ##
+      # Adds after_commit
+      def after_commit(*args, &block)
+        options = args.pop if args.last.is_a? Hash
+        if options
+          case args[:on]
+          when :create
+            after_create(*args, &block)
+          when :update
+            after_update(*args, &block)
+          when :destroy
+            after_destroy(*args, &block)
+          else
+            after_save(*args, &block)
+          end
+        else
+          after_save(*args, &block)
+        end
+      end
 
       ##
       # Adds Mongoid::Paperclip's "#has_mongoid_attached_file" class method to the model