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 
Should we put the `identify` call after `.page()`? Not 100% sure but seems like we should, since I think the default script is do call initialize/page, then identify. So maybe:

```js
analytics.initialize();
analytics.page();
analytics.identify('user-id');
```