Migrate from nREPL.el to CIDER

Basics steps:

 * Uninstall nREPL.el package
 * Rename nrepl-* to cider-* (as needed)
 * Upgrade clojure-test-mode
 * Install CIDER

Some things are still named nrepl-*, so I grepped the CIDER code base to
see which things needed to be named cider-*. Also removed Slamhound as
part of this, since I wasn't using it.