EPUB writer:  Include html TOC even in epub2.

* 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.