Import a database array as a collection response

It would be nice to do something like this:

``` php
$results = 'mysqli_query() + fetch_assoc() while-loop';
$jsonapi = new jsonapi\collection($type='users');
$jsonapi->import_from_array($results);
```

And maybe a way to split keys as relationships. Like `import_from_array($array, $split_relation_names)` or `$array = collection::split_relations($array, $relation_names)`.
