Add convenience bindings to clojure-mode-map for starting nrepl

Since people are generally in some Clojure buffer when starting
nrepl.el it's useful to have convenient keybindings for that in
`clojure-mode-map`. The mnemonic for the keybindings are `j` for jack-in
and `c` for connect. The keybindings are autoloaded and added to
`clojure-mode-map` after `clojure-mode` has been loaded.