Improved user experience

As a long time SLIME user I welcome this new nrepl tool for Emacs. Over the years I got used to some keybindings, so I add them here. Also, SLIME got rid of those connection buffers, which I did here too. It is a bit bothersome to navigate through the buffers (for example via `C-x <left/right arrow key>`) and have the *nrepl-server* or *nrepl-connection* buffer. Those are now hidden.
And as `nrepl-jack-in` is a bit lengthy, I added the `clj` alias.