Add feature: switch back to clojure buffer

Try to remember the clojure buffer before
nrepl/nrepl-jack-in/nrepl-swith-to-nrepl-buffer,
so that the user could jump back to the original
clojure buffer from the repl buffer. The key binding
is also C-c C-z, which makes it convenient to
jump between clojure and repl buffers.
Merge pull request #344 from moonranger/switch-to-clojure-buf-squashed

Add feature: switch back to clojure buffer