Add support for nested and object attribute types

[demsullivan]

This PR adds support for specifying nested or object attribute types in ROM. As an example:

class User < ROM::Relation[:elasticsearch]
  schema do
    attribute :profile, ROM::Elasticsearch::Types.Nested do
      attribute :email, ROM::Elasticsearch::Types.Text
    end
  end
end

Using ROM::Elasticsearch::Types.Object in the same fashion will create an object field in the Elasticsearch index mapping.

Fixes #17