remove dependency on GutenTag

[aparrish]

GutenTag is an amazing project but I really only used its code and corpus as a way to quickly "bootstrap" the necessary code and files. I don't think it's a sustainable foundation moving forward, especially for keeping the files in this corpus up-to-date with the latest releases (and metadata changes) on Project Gutenberg itself. My best idea so far is to set up a Project Gutenberg mirror and modify the code to work directly on the files from the mirror, but that obviously takes a lot of effort (and hard drive space). Open to other suggestions.