Ebookconverter

- Removed a txt exclusion. Ebookmaker is now putting headers on txt files, but this was not being done for the 20,000 or so books with source named 99999-0.txt due to the obsolete exclusion.
- add a script that packages txt files to a tarball
- add an HTMLWriter that extends Ebookmaker's HTMLWriter to package up a zip file of the html5 with images named pg99999-h.zip Addresses 38
- don't require a sourcefile for products that are built from the database. Require ebookmaker>0.12.35. Fixes 39.
- save credit metadata to the database. fixes 28.
- corrected behavior of source file size limits
- add ebook number to log even if the source file fails to parse

- adds MastodonWriter, to replicate TwitterWriter (posting on gutenberg_newmastodon.social)

- normalize capitalization of "eBook"
- only send notification for CRITICAL errors 33

- notifications, both success and problem alerts, are now sent only to the whitewasher.
- notification texts have been cleaned up to be more readable and more friendly
- refactored PUBLIC, FILES, FTP into libgutenberg>=0.10.15
- CACHEDIR and FILESDIR now need to be set in ~/.ebookmaker as well as ~/.ebookconverter, Probably these belong in an environment variable.

- added documentation for commands
- add checking that a source file exists
- refactor filesize checking to remove CRITICAL messages when a build is skipped
- update tests

- Adds UpdateWriter. It scans source files for metadata headers and adds to the database. Currently only looks for 'Credit:' or 'Produced by:'.
- `.xhtml` added to parseable file extensions
- stop producing kindle.noimages unless asked to
- add a "generic" file name for Writers that don't write (Twiter, Facebook, Update)