Move iswitchb.el to obsolete/

Move iswitchb.el to obsolete/

Ref: http://lists.gnu.org/archive/html/emacs-devel/2013-11/msg00501.html

* lisp/obsolete/iswitchb.el: Move to obsolete/.

* lisp/simple.el (iswitchb-mode): Add manual autoloads to ease transition,
since obsolete/ is not scanned for autoloads.

* lisp/emacs-lisp/authors.el (authors-valid-file-names):
Add iswitchb.el.

* doc/emacs/ack.texi (Acknowledgments): No longer mention obsolete files.

* doc/misc/viper.texi (File and Buffer Handling): Prefer ido to iswitchb.

* etc/NEWS: Related edits.