Autoload sx-authenticate

This also removes the sx-authenticate alias and renames
sx-auth-authenticate to sx-authenticate.

I did this because I felt it wasn't helpful to have both. Whenever I hit
`M-x sx-au <TAB>` the only completions offered were two identical, but
differently named, commands. That's useless at best, confusing at worst.
Merge pull request #147 from vermiculus/autoload-auth

Autoload sx-authenticate