reduce 2x map; fixes datamapper eager loading
Merge pull request #210 from tpitale/master

Reduce double map to once; Fixes datamapper eager loading.
Add ActiveSupport::DescendantsTracker to Serializer and ArraySerializer
Remove caching of Serializer constants to allow ActiveSupport::Dependency reloading to work
Merge pull request #215 from mulder/feature/reloading

Serializer Reloading via ActiveSupport::DescendantsTracker
changes to be able to specify multiple attributes with keys