Requested/noticed in https://github.com/open-source-parsers/jsoncpp/pull/610, and a
followup to https://github.com/open-source-parsers/jsoncpp/pull/607 .

Google advises its employees to add Google Inc. as an author, but that hasn't
been done yet and would be super inconvenient. So instead I've refactored the
file to refer to "The JsonCpp Authors", which are listed in the AUTHORS file.

The AUTHORS file itself is generated via:

    git log --pretty="%an <%ae>%n%cn <%ce>" | sort | uniq

Plus the addition of "Google Inc." as a copyright author. (Google owns the work
of anyone contributing from an @google.com address, for example.)

The list contains some probable duplicates where people have used more than one
email address. I didn't deduplicate because -- well, who's to say they're
duplicates, anyway? :)

JsonCpp is now licensed under MIT license, or public domain if desired and recognized in your jurisdiction.

Removed experimental notification on iterators, and added experimental status for allocator (to be removed)