def initialize(comments, options={})
      raise ArgumentError, "A comment needs a parent comments object" unless comments.is_a?(Comments)
      @visible = true
      @comments = comments
      parse_options options
      yield self if block_given?
    end
Creates a new comment object
@param [Comments] comments The comment collection this comment belongs to
@param [Hash] options
@option [String] author the author of the comment
@option [String] text The text for the comment
@option [String] ref The refence (e.g. 'A3' where this comment will be anchored.
@option [Boolean] visible This controls the visiblity of the associated vml_shape.