Using tags as an alternative way to populate the folders attribute

Hi.
I was trying to use this gem to parse bookmarks from a delicious export and noticed that bookmark.folders came out empty in every bookmark.

Since tags could actually be used as folders in delicious, I made these changes so I can get them when accessing the folders attribute, whilst not ruining your method of getting folders (using h3).

Hope this is useful for someone else. Thanks for the cool gem.