Makefile: fix quoting

You don't need to quote Makefile variables, and the quotes around
"$(FILES)" in the shell loop expanded it to:

  ""Readme.md extension.js metadata.json edit-groups""

So the two errors cancelled each other out.
Makefile: add dist and clean targets
Add Günther Wutz to AUTHORS
Include COPYING etc in distributed zip file.
Add a new-school example file.
Makefile: refuse to make dist in a dirty tree
Restructure README
Ignore tarballs