v0.3.3.3 gateway tweaks

The gateway now advertises itself as v0.3.3. Otherwise, v0.3.3 nodes will refuse to connect to it. This is a temporary measure to alleviate network splitting.

Nodes are pruned by makeOutboundConnections if they aren't reachable. If the connection fails for a different reason, the node is not pruned.

The ExternalIP function now times out after 3 seconds. This is the easiest way to prevent clients without Internet access from hanging. I'll implement a more robust solution when I tackle UPnP, which gives us an alternative way of discovering the external IP.