Correct README.md and add autoload cookies.

* README.md: Correct for autoloads.

* sly-company.el (company-sly, sly-company-mode): Add autoload cookie.
Fix bugs

* sly-company.el (company-sly, sly-company-mode): Add autoload cookie.
(sly-company): Rename from `company-sly. Don't use `case'. Ignore
`post-completion` action.
(cl-lib): require it.
Also add Emacs 24.3 to "Package-Requires"