Rework and and simplify setup instructions

* slime-tests.el (slime-test-recipe-test-for): new function for
testing setup recipes.
(readme-recipe): new test;

* slime.el: don't run `slime-load-hook'.
(slime--setup-contribs): Hide from user. Idempotent.
(slime-setup): Now a command.
(slime-contribs): New variable.
(slime-setup-contribs): Obsolete this variable to point to
`slime-contribs'.
(defun slime-setup-contribs): use `slime-contribs' var.

* README.md: show travis build status for master branch only.
(Quick setup instructions): mention `slime-autoloads' and suggest
adding contribs to load path. Suggest `eval-after-load'.
(Contribs): new section.
(Emacs 23 gotchas): new section.
(Contact): update mailing list info and link to CONTRIBUTING.md.

* slime-autoloads.el: simplify with a `eval-after-load'.

* slime.el: Bump version to 2.3.2

* contrib/README: Renamed to README.md.
* contrib/README.md: New file. Don't mention setup technique.