def invite(user)
      return false if user == self || find_any_friendship_with(user)
      Amistad.friendship_class.new{ |f| f.friendable = self ; f.friend = user }.save
    end
def invite(user)
      return false if user == self || find_any_friendship_with(user)
      Amistad.friendship_class.new{ |f| f.friendable = self ; f.friend = user }.save
    end
suggest a user to become a friend. If the operation succeeds, the method returns true, else false