Various fixes

(In connection with https://github.com/milkypostman/melpa/pull/1132)

* Fix header and trailing lines
* Add missing keywords header
* Require 'cl at runtime, not compile-time, for delete-duplicates
* Only use `' quoting for lisp symbols
* More closely follow checkdoc guidelines for docstrings
* Add missing defvar for command history

In general, you should enable `auto-insert-mode', which will prompt you to interactively insert a standard skeleton for new emacs lisp files -- the resulting layout will be automatically compatible with `package.el`, which this was not. Also, turn on `checkdoc-mode` to get nagged about docstrings. It helps make sure you follow the guidelines for elisp documentation.

Regarding the code itself, you might consider using John Wiegley's `alert` library, or even `todochiku` in order to display platform-aware notifications.

Cheers!

-Steve