add EPUB3 HTML TOC support

Parses the EPUB3 HTML Navigation document (if available) instead of the EPUB2 NCX file.

Attempts to recreate the same tree structure as `Parser.toc`.

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This Pull Req seems to get the https://github.com/philschatz/epub-anatomy book to load in epubjs-reader; I did not update the `gh-pages` branch because:

1. I do not know how to rebuild the `epub.min.js` file inside `epubjs-reader` (I added a reference to underscore in the `index.html` and pointed it to epub.js instead of epub.min.js)
2. I do not know the quickest way to add epubjs-reader to the `gh-pages` branch

Also, I noticed the ToC links have relative hrefs to the XHTML files but I think I am missing code to absolutize the hrefs.