Better logging

Had a bug in TestKit. To resolve it i needed full debugging, but there was a lot that was missing so I added this:
- Log a debug message when system is shutting down
- Better Mailbox debug logging
This applies only to when compiling Akka with `MAILDEBUG` flag
- Override ToString for SystemMessages for more info when logging. The only time system messages are logged is when  `MAILDEBUG` flag is set.