Update NEWS.txt for 2.8
Increase version to 2.8.
Prepare NEWS and README for 2.7 release.
Mention that adadomain is no longer needed.
Update NEWS.

Change the release year, it didn't happen in 2017.
Also, mention the Adacontrol rule updates.
Mention Set_Up/Tear_Down changes.
Bump release year forward.

I didn't manage to do a release in 2016, maybe I have better luck this year.
Mention Bounded_String to Unbounded_String change in NEWS.
Change gnat/gnat_gen/gnat_linux in NEWS.txt to match the situation.
Mention the latest changes in NEWS.
Mention LICENSE.txt renaming.
Start NEWS section for Ahven 2.7.
Rename NEWS to NEWS.txt.

Makes it easier to figure out that the file is a text file.