Give option to set the name of a property

Sometimes it may be useful to have the ability to set the name (the name of the attribute of the object you are representing) in the options passed into a Definition. 

    class Article
      include Representable::JSON
      attr_accessor :title
      property :heading, :name => :title # implicit :from => :heading
    end

    a = Article.new; a.title = "Godzilla!"
    a.to_json # => '{"heading" : "Godzilla!"}'