def initialize(options={})
      @workbook = nil
      @core, @app = Core.new, App.new
      @core.creator = options[:author] || @core.creator
      @core.created = options[:created_at]
      parse_options options
      yield self if block_given?
    end
Initializes your package

@param [Hash] options A hash that you can use to specify the author and workbook for this package.
@option options [String] :author The author of the document
@option options [Time] :created_at Timestamp in the document properties (defaults to current time).
@option options [Boolean] :use_shared_strings This is passed to the workbook to specify that shared strings should be used when serializing the package.
@example Package.new :author => 'you!', :workbook => Workbook.new