Updates to prepare for the 0.104 release candidate:

- Change documentation to explain current bytecode runtime situation.

- Document Python 2 pytest issue.

- Add additional contributors to acknowledgements.

- Update Install instructions to note that Autotool has been removed.

- Add *.cat SHA256 support and PDF bytecode hook bugfix to the News.

- Clarify purpose of the clamscan `--gen-json` option in the
  clamscan --help.