Add RPC to show current total available channel capacity #29 (#35)

This commit adds a new RPC command: `channelbalance` which returns the
sum of all available channel capacity across all open channels. The
total balance is currently returned in units of `satoshis`. Additionally
the `networkHarness` has been modified slightly to allow specifying the
additional "extra" command line parameters when creating the initial
seed nodes. Minor refactoring within the integration tests has been
undertaken in order to increase code re-use across tests.

Closes #29.

cmd/lncli/main.go

@@ -56,6 +56,7 @@ func main() {
 		CloseChannelCommand,
 		ListPeersCommand,
 		WalletBalanceCommand,
+		ChannelBalanceCommand,
 		ShellCommand,
 		GetInfoCommand,
 		PendingChannelsCommand,