Tidying up switch-to-relevant-buffer.

Hi,

What's in here:

1) cider-switch-to-relevant-repl-buffer is mapped to C-c C-z. I've tidied related keybindings, and specially removed the ability to rotate and to display the current nrepl connection from the REPL mode (still exists for CIDER mode). It makes little sense to do it here, and generally I think it's good to prune where possible.

2) cider-switch-to-relevant-repl-buffer again takes 2 prefix args (first one does the namespace switch, second one triggers IDO to select project).

3) Switch will NOT be made based on project dir if there exists a REPL connection without a project dir. If this is so, then a message is printed.

4) Better message output when a switch is made.

Some notes:

1) I've kept cider-switch-to-repl-buffer around. This is because it's quite discreet in what it does and it does offer users an option if they dislike the funkier behavour. In time someone might remove it.

2) I've tried to make "Managing multiple sessions" in the readme a little more easy going. There are issues around the precise terminology to use, like "CIDER minor mode" vs "Clojure buffer", or "current nrepl connection" vs "default nrepl connection", "switch vs select".