Graceful shutdown

siad no longer terminates via os.Exit, but simply by returning. It accomplishes this by waiting on a channel. It can also be killed via interrupt, and will attempt to shutdown cleanly. I'd like it to save the wallet file before closing; however, the Wallet interface no longer has a Save method, so right now there's no way to do it cleanly. (It can be done "uncleanly," via type assertion, but I opted not to do so.)