doc: make tools/update-authors.sh cross-platform

And by cross-platform I mean Linux and OS X.  The awk script is not
compatible with BSD awk, that's why this commit changes it to perl.

Update the .mailmap to remove some duplicates and regenerate the
AUTHORS file.

Fixes: https://github.com/iojs/io.js/issues/1120
PR-URL: https://github.com/iojs/io.js/pull/1121


Reviewed-By: Rod Vagg <rod@vagg.org>