Add autoload cookie and footer line for ELPA installation

Hi, I plan to add this to [MELPA](http://melpa.milkbox.net/). 

`autoload cookie` ensures that users who have installed `sos.el` from [MELPA](http://melpa.milkbox.net/) will be able to use the code without explicitly requiring the library first. 

Footer line makes `sos.el` installable with `package.el`. 

For information about this commit, See following GNU Emacs Manual:
- [Multi-file Packages](http://www.gnu.org/software/emacs/manual/html_node/elisp/Multi_002dfile-Packages.html)
- [Library-Headers](http://www.gnu.org/software/emacs/manual/html_node/elisp/Library-Headers.html)