[core] Adds #persisted? to Fog models

Fog models have been able to use #new_record? for years to use an
ActiveRecord like interface to check for peristence of a resource.

When Rails 3 switched to using ActiveModel API this changed to
be #persisted?

This deprecates #new_record? and adds #persisted? as the first little
step to making fog models easier to use using ActiveModel API.

As discussed on the tickes, ActiveModel is not planned to become a
direct dependency for fog.

Relates to #1276

lib/fog/core/attributes.rb

@@ -152,8 +152,25 @@ module Fog
         self
       end
 
+      # Returns true if a remote resource has been assigned an
+      # identity and we can assume it has been persisted.
+      #
+      # @return [Boolean]
+      def persisted?
+        !!identity
+      end
+
+      # Returns true if a remote resource has not been assigned an
+      # identity.
+      #
+      # This was added for a ActiveRecord like feel but has been
+      # outdated by ActiveModel API using {#persisted?}
+      #
+      # @deprecated Use inverted form of {#persisted?}
+      # @return [Boolean]
       def new_record?
-        !identity
+        Fog::Logger.deprecation("#new_record? is deprecated, use !persisted? instead [light_black](#{caller.first})[/]")
+        !persisted?
       end
 
       # check that the attributes specified in args exist and is not nil