Add autoloads, format header more idiomatically

Couple of minor fixes following request for MELPA inclusion.

I've also changed the suggested keybinding to `C-c C-z` which is used by slime, nrepl and others: it's pretty much the standard key for "switch to repl", where such functionality is available.

BTW, I'd also strongly suggest dropping the hash-based scheme with "add" functions: it would be much more idiomatic and convenient to have a `defcustom` variable which is an `alist` mapping major modes to functions.

-Steve