Fixes to IV

Added identify method (maps to 'setUserId') per Enriko's [request](https://segment.zendesk.com/agent/#/tickets/17101), as well as a test to check for userId. @lancejpollard @yields 
maybe we should do a check here?

```js
var userId = this.analytics.user().id();
if (userId) push('setUserId', userId);
```