Finish release of BNFC-2.8.4

[andreasabel]

Release BNFC-2.8.4:

• [x] finish testing
• [x] check documentation for outdated infos
• [x] write release notes
• [x] upload to hackage
• [x] v2.8.4 release tag
• [x] github release
• [ ] linux binaries
• [x] win binary
• [x] mac package
• [x] readthedocs
• [x] update bnfc webpages https://github.com/BNFC/bnfc.github.com
• [x] bump master to 2.9.0
  • [x] source/changelog
  • [x] source/BNFC.cabal
  • [x] testing/bnfc-system-tests.cabal
  • [x] docs/conf.py

[andreasabel]

Linux binaries are still missing. TODO:

• [ ] investigate how the installers worked for previous versions
• [ ] build at least the 64bit binaries