Use repl-buffer rather than nrepl-buffer

The use of nrepl-buffer seems ambiguous, so rename to repl-buffer.

Renames to nrepl-repl-buffer, nrepl-create-repl-buffer,
nrepl-check-for-repl-buffer, nrepl-current-repl-buffer.
Allow connections without repl buffers

In order to support tools like lein.el, which use nrepl.el pragmatically, add
the ability to connect without creating a repl buffer.