Add -pkg.el file

Hi, I plan to add this to [MELPA](http://melpa.milkbox.net/), This commit makes `ztree` installable with `package.el`. Feel free to change a description if desired. For information about this fix, See GNU Emacs Manual, [Multi-file Packages](http://www.gnu.org/software/emacs/manual/html_node/elisp/Multi_002dfile-Packages.html).