Allow HTTP options to be set client-wide

This allows HTTP options to be set globally across all soap requests to avoid repeating them:
```
soap.createClient(url, function(err, client) {
    client.setHttpOption('timeout', 10000) // sets the timeout across all requests

    client.MyFunction(args, function(err, result) {
        // will use a 10 second timeout 
    });
});
```