Forcing api_mapping configuration to be inherited to child classes

With that change, it will be possible to have a parent class containing
default configuration for all the models of a certain API.

For example:

module GooglePlusAPI
  class Base
    include Jeckle::Resource

    default_api :google
  end
end

module GooglePlusAPI
  class Person < Base
    # will already be mapped to API `:google`

    def search_by_name
      # ...
    end
  end
end
Making sure a inherited jeckle resource does not affect the parent's api mapping