Rework and simplify setup instructions

* slime.el: don't run `slime-load-hook'.
  (slime--setup-contribs): Hide from user. Users should set
  `slime-contribs' instead.
  (slime-setup): Now a command.
  (slime-contribs): New variable.
  (slime-setup-contribs): Obsolete this variable to point to
  `slime-contribs'.

* 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.

* slime-autoloads.el: simplify with `eval-after-load'. Don't
  provide `slime-setup' anymore.

* slime.el: Bump version to 2.3.2

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