Namespaced serializers #499

So, here's what I did.

+ Serializers for associations are being looked up in a parent serializer's namespace first.
+ Same with controllers' namespaces.
+ Added a "prefix" option in case you want to use a different version of serializer. (For example, I prefer making lighter versions of serializers for associations, has_many :users, prefix: :short -> renders ShortUserSerializer).

@steveklabnik 
This overwrites any custom namespace defined in `default_serializer_options`.