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 better logging
Merge pull request #419 from HCanber/better-logging

Better logging
- 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.