Keybinding improvements

* Bound `nrepl-interrupt` to `C-c C-c` in REPL buffers, to respect Emacs comint tradition.
* Bound `nrepl-load-current-buffer` to `C-c C-c` to respect the common practice to use this keybinding for
a very popular action. Arguably in nrepl nothing is as popular as loading the current buffer.
* `nrepl-switch-to-repl-buffer` now accepts a prefix argument. If one is passed it will take into account the
namespace of the current Clojure source file.
Fixed a typo.
Rebind `C-c C-c` to nrepl-eval-expression-at-point.
Merge pull request #273 from bbatsov/keybindings

Keybinding improvements