[emacs mode] migrate from cl.el to cl-lib.el

cl.el has been deprecated and cl-lib.el is the recommended choice
for Common Lisp support within Emacs which properly namespaces all
new symbols (cl-*).

Migrate both tern-auto-complete.el as well as tern.el.

Users of Emacs <24.3 can use the cl-lib.el shim from ELPA which
should be automatically installed if tern is installed through an
appropriate package repository like MELPA.

This also fixes issue #320 where the function call signature help
was broken due to CL "find" being unavailable during runtime if
cl.el was not globally available (which is not recommended).

Fixes issue #320.