Simplify handling of the ssh tunneling

   - remove configurable nrepl-connection-endpoint
   - nrepl-connection-endpoint is now a function that returns the endpoint (either ssh or plain)
   - nrepl-on-connection-buffer is now nrepl-tunnel-buffer
Merge pull request #756 from vitoshka/ssh-tunnel

Rename on-connection-* into tunnel-* and simplify related code.