core: add link to formatted version of {ChangeLog|NEWS}.asciidoc
relay: add signals "relay_client_xxx" for client status changes (closes #2)

New signals:
- relay_client_connecting
- relay_client_waiting_auth
- relay_client_connected
- relay_client_auth_failed
- relay_client_disconnected
relay: add info "relay_client_count" with optional status name as argument

Use in evaluated strings (for example in /eval or triggers):
- "${info:relay_client_count}": total number of relay clients (any status)

With a specific status:
- "${info:relay_client_count,connecting}"
- "${info:relay_client_count,waiting_auth}"
- "${info:relay_client_count,connected}"
- "${info:relay_client_count,auth_failed}"
- "${info:relay_client_count,disconnected}"