Allow switching between networks + network version checks

[gak]

Right now the node connects to the live network only. It would be good to be able to connect to the test and beta networks.

Possibly using a --network argument if one isn't already set.

Maybe if possible, create a feeless test network too.

This would mean peering.nano.org would have to change depending on the network.

This spills into making sure that node handshakes verifies the network and versions correctly, because the beta network runs on a newer node version (and I think network version too).