Ebookmaker

- refined embedded media error message to include referrer
- added external link warning

- Ebookmaker has been downloading and including in epubs embedded media from arbitrary websites, for example, images. This has caused build errors when a remote site goes away. ebookmaker has command line parameters that allow include and exclude urls that have governed document files; these same rules now apply to media files.

In general, PG books should not embed media from other sites.

Support docutils-0.8-style :class: language-<code>.

- support ebook-convert tool from Calibre as alternative to kindlegen

To use calibre instead of kindlegen

1. install calibre
2. change MOBIGEN setting to 'ebook-convert' or the path to ebook-convert

- build failures on our production system suggest that the parser's output for empty style elements may be os-dependent. Parser now handles style elements with null text.
- allow the build to succeed even if a css file is missing
- allow px only in border properties. see discussion below
- fix false encoding error (complaining about "Klingon") when content file is missing

Ebookmaker has been removing any css rules that use 'px' measurements. 'px' measurements are discouraged in css styles for epub because of poor scaling when user changes font size. However fixed lengths are still useful in properties such as table borders. see discussion on DP forum:
https://www.pgdp.net/phpBB3/viewtopic.php?f=3&t=41237&p=1188773&hilit=windymilla#p1188746

- Ebookmaker was failing to make epubs if a linked file was missing. Now it emits an Error message to the log with url of missing file, but goes ahead and makes an epub without it.