Add support for singletons to Acfs

Usage:
  Single.find
    # sends GET request to http://service:port/single

  my_single.save
    # if my_single is a new object:
    #   sends POST request to http://service:port/single
    # else if my_single has been requested before:
    # sends PUT request to http://service:port/single

  my_single.delete
    # sends DELETE request to http://service:port/single

SingletonResources do not support the Resource method :all, since
always only a single instance of the resource is being returned