Add/array support

adds support for arrays as `options.evens`, for example:

```js
events = [
  { key: 'event', value: 1 },
  { key: 'event', value: 2 }
];
``` 

in this case doing `.track('event')` will track twice, once for `1` and then for `2`.

cc @ianstormtaylor @lancejpollard 
maybe we should isolate the `window.__adroll.record_user(data);` to one place (same for all integrations), so it's easy to see where the external API calls are actually made, and eventually we can potentially abstract that further.

so maybe, creating an array of data objects, then just iterating through the array to calll `window.__adroll.record_user(data);`?