raven.transport.threaded spams messages to stdout

`raven.transport.threaded` sends messages to stdout on shutdown. This means that raven cannot be used in situations where you care about what gets sent to stdout. At the very least this needs to be made optional. Perhaps it should be made conditional on `sys.stdout.isatty()` - the output about "Press Ctrl-C to quit" makes no sense anyway if it isn't a tty.