Switch autotools to "foreign"; cleanup NEWS

Running autotools in "gnu" mode comes with significant constraints:
- shipping a a README file, which we previously hacked around
- a predefined INSTALL
- a ChangeLog in a well-defined format which we currently fail

Thus, switch to "foreign" and ship what we actually want with
dist_doc_DATA. While at it, merge RELEASE-NOTES and ChangeLog into NEWS
and remove all the "see RELEASE-NOTES" boilerplates that we had.
Update NEWS with 5.0's license change
Update .gitignore: add tarball's name
Kill contrib/ and tests/
Minor autotools update

Use tar-ustar because it's 2014 and remove a redundant INCLUDES that
newer automake complains about.
Update README.md

Format in Markdown, plus various content changes.