wns: an option object can optionnaly be passed to wns lib

For some notifications, options need to be set.
For example, to send a notification of type 'raw' the 'Content-Type' must be set to 'application/octet-stream'
```js
var options = _.defaults({
  client_id: data.client_id,
  client_secret: data.client_secret
}, data.options);
```