Added active model serializer support for Tire results

Example:

    require 'tire/model/active_model_serializer'

    class Vacancy
      include Tire::Model::Search

      # ...
    end

    class VacancySerializer < ActiveModel::Serializer
      attributes :position, :body, :location

      def location
        "#{city}, #{state}, #{country}"
      end
    end

    class VacanciesController < ApplicationController
      def index
        render json: Vacancy.search(params[:term]), root: false
      end
    end

See: https://github.com/rails-api/active_model_serializers

Closes #15