Rework and 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.
(Contact): update mailing list info and link to CONTRIBUTING.md.

* 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.

* contrib/slime-repl.el (slime-repl-read-mode): Closes #17: ignore
slime completion `slime-repl-read-mode-map'.
(slime-repl-read-mode-map): Put keymap in its own var.