add AUTHORS to MANIFEST.in, so it gets included in releases

MANIFEST.in

@@ -1,4 +1,4 @@
-include README.rst LICENSE CHANGES MANIFEST.in versioneer.py
+include README.rst AUTHORS LICENSE CHANGES MANIFEST.in versioneer.py
 recursive-include borg *.pyx
 recursive-include docs *
 recursive-exclude docs *.pyc