Ebookmaker

- 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.

- updated libgutenberg to 0.5.1
- added warning when mediatype of a linked file cannot be determined

- fixed bugs in ParserFactory that raised exceptions for parsers built from package-supplied
resources
- fixed bug that raised exceptions when a cover was rejected for being too small

- disabled html to txt conversion because no conversion was being done
- changed a truth eval of an etree element to silence deprecation warnings
- corrected documentation of config behavior
- better install documentation