add autoload cookies and refine source code.

Thank you for your work.
I send a pull request.

* For MELPA users, it is useful if autoload cookies are inserted in the source code, because `(package-initialize)` creates corresponding autoload functions.
* Your `psw-get-buffer-list` is somewhat redundant. `remove-if` can be used. Also, `buffer-live-p` is also redundant because `(buffer-list)` returns all existing live buffers.