Closes #112: Clarify use of slime-load-hook, don't use eval-after-load

* slime-autoloads.el (slime): Don't use `eval-after-load'.

* slime.el: Call `slime-setup' at the end.

* doc/slime.texi (Installation): Make the autoload recipe the
default recipe.
(Basic customization): New subsection. Mention `slime-load-hook`
and techniques for customizing keymaps.