Instead, just use an a element with class `footnoteRef`.
This allows more styling options, and provides better results
in some readers (e.g. iBooks, where anything inside the a
tag breaks popup footnotes).
Closes#1995.
* The TOC is included in `<spine>`, but `linear` is set
to `no` unless the `--toc` option is specified.
* Include `<guide>` element in OPF.
* This should allow the TOC to be useable in Kindles when
converted with kindlegen.
* Results validate with epubcheck 3.0 for both epub and epub3
output.
* Closes#773.
