give option to supply a Proc to a Definition that acts on the object

Give the option to supply a Proc to a Defintion that acts on the object
being represented.

    class SensationalArticle
      include Representable::JSON
      attr_accessor :title

      property :title, :proc => Proc.new {|article| article.title + "!"}

      property :lede, :writeable => false, :proc => Proc.new { |article|
            "Experts are warning that " + article.title.downcase + "!!"
      }
    end

Now let's use it. 

    a = SensationalArticle.new
    a.title = "Bees Are Found To Make Honey"
    a.to_json

Yields

    {"title" : "Bees Are Found to make Honey!",
     "lede"  : "Experts are warning that bees are found to make honey!!}