Please support multiple Growl hostnames

I want to send Growl notifications to multiple computers (one at work, one at home), so I added support for it.

I had to remove the test_connection() and warning when the Growl host is unresponsive. Having irssi-growl warn every time a host is unreachable gets really noisy when my work computer is off, or vice versa. I couldn't come up with a good way to keep it; maybe only warn every N messages? Seems like that might still result in a lot of noise.

I also added support for a timeout setting (default: 2 seconds) since the default Growl::GNTP/IO::Socket timeout caused irssi to block for too long when a host is unreachable.