Serializer Reloading via ActiveSupport::DescendantsTracker

Currently serializers will not reload (#132) when working in Rails development mode.  This is makes it work.

fa51c5f allows serializers to be reloaded.

Currently ActiveModel::SerializerSupport caches constants which gets in the way of the way the rails reloading works.  918a9de disables the problematic caching. 

If we need to keep the caching for performance reasons we can do so in a way that works with the rails reloader.  We would need a way to clear out these cached values when the reloader is told to cleanup.  Storing them in a simple store (hash) maybe a good place to start.