Keybinding improvements

* Bound `nrepl-interrupt` to `C-c C-c` in REPL buffers, to respect Emacs comint tradition.

We get a nice mnemonic - `C-c` is interrupt in Unix shells. We also get consistency with just about every other comint-style mode for Emacs. And we keep `C-c C-b` for another layer of consistency.

* Bound `nrepl-load-current-buffer` to `C-c C-c` to respect the common practice to use this easily accessible keybinding for
a very popular action. Arguably in nrepl nothing is as popular as loading the current buffer.

'Compiling' defns makes little sense in Clojure, but using the keybinding for loading the current buffer is pretty sensible I think. And the keybinding is much easier to press than `C-c C-k`. 

* `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.

Since we're altering the behaviour of the switching to the repl command it makes sense for it to take a prefix argument (instead of the namespace changing command).

PR is influenced by the discussion that took place on #235.

This supersedes #234.