Added hidden_attributes method for default hidden attributes.

Adds the ability to define `hidden_attributes` that are not rendered by default. For example, say you have a `UserSerializer` with a `home_address` attribute that you don't want to remember to exclude every time you use that serializer. Just define your serializer with `hidden_attributes :home_address` and then add the `expose: :home_address` option in the places where you do want `home_address` displayed.