alertnotify, so bitcoind users can get email/sms/whatever of alerts

This gives people running bitcoind a way of getting alerts in a timely manner, instead of needing to poll with "getinfo" and parse the "errors" array.

Example usage in a bitcoin.conf:

    alertnotify=echo %s | mail -s "URGENT: bitcoind alert" admin@service.com

The command string is passed to the shell, with %s replaced by the alert message enclosed in single quotes.