Asynchronous notification

[KevinLai]

It would be useful to have asynchronous notification of various events and state changes. For example, uses would like to be notified if a host account resource is likely to fall or has fallen below a certain threshold. Users could ask for notification if the likelihood of a resource falling below a minimum is exceeded. In particular, users could determine if their VMs are in danger of being suspended or deleted. It would also be useful to have multiple notification transports: email, IM, RPC, etc.