New notification system:
You can specify an external program that will be called in case of alerts and other system critical events
It is invoked through Node::notify, and can e.g. be specified on the commandline by the notify option as:
	--notify="/usr/bin/mail -s libcoin_issues admin@mysite.com”
The external program get the notification on stdin.
The default notifications are alerts and if an invalid chain becomes longer than our main chain.
You can in a daemon specify other notifications (like e.g. if your wallet is running out of funds or if a transaction is stuck in confirming)
The design of the notification system is tied into the Filters - so any filters now supports a hook into the notification system that is again controlled by Node.

Also added a BlockChain::confirmations method taking a tx_id hash and returning the number of confirmations.