Added -txnotify to call a script when a transaction is received

More or less copied code from the -blocknotify and -walletnotify, but for all accepted transactions.

Is there a good reason not to implement this? As far as i can tell any DOS attack will be limited by the rate limiting of free transactions and even then process spawn limits can be set on any unix system.

As long as the script does something simple, such as adding the txid to a queue, then there will be little chance of the number of spawned processess building up. This is very useful to people who would otherwise have to rely on constant polling or centralised services such as blockchain.info websockets.