Drop http-post-simple and use emacs-web

By dropping http-post-simple, paste-kde doesn't need to distribute the
library and rely on the user installing emacs-web.
Add packaging bureaucracy

Also bump version of Emacs required to 24.
Use custom find function
Add option to save url to the kill ring
Drop the need for json using `web-json-post'
Bump the version of emacs-web
Mark lexical variable as unused