AUTHORS list updated for 2.15

[Graham Percival]

I've moved all contributors' names from the "current" section to
the "previous" section.  I added a few names I saw in the git
changelog as coming after release/2.15.0.  We'll continue to add
names to the "current" section as patches get accepted to git
master.


Note that right now, there are no new translation patches in git
master.  The translators could start adding their names to
  Documentation/included/authors.itexi
on their own branch and then it'll get merged with master when
that next happens?

For that matter, Francisco should decide if he wants to keep
lilypond/translations merged with stable/2.14, or whether he wants
to merge it with master.  My 2 euros suggest that he keeps it with
stable/2.14 until a few weeks before the first 2.16 release
candidate... but honestly, there will be problems no matter which
way it goes.  Branching is icky.

Cheers,
- Graham