No way to batch requests

It would be good to expose the `batch` endpoint. Ideally something like this:

``` php
$create_users = [
    'rid1' => ['first_name' => 'John', 'last_name' => 'Doe'],
    ...
];
$responses = $wheniwork->batch('post', 'users', $create_users);
```

When the `$responses` come back, I would like to have it indexed by the keys of requests, rather than assuming FIFO ordering.
