Add feature: switch to clojure buffer

It's very convenient to use <kbd>C-c C-z</kbd> to jump from a clojure buffer to repl buffer. It is even sweeter  if we can use the same key to jump back, just like [geiser](http://www.nongnu.org/geiser/) does for scheme-mode.

This pull request adds this feature to nrepl.el. I think it's useful, how do you think?