allow multiple server URLs in the example code

this patch makes the examples work with a clustered gnatsd setup.

this is how i invoke them:

```
go run nats-pub.go  -s nats://localhost:4245,nats://localhost:4242 key.abc hello-world
go run nats-sub.go -s nats://localhost:4242,nats://localhost:4245 \*.\*
```