Add generator for large datasets

[barryvdh]

By using yield, we can use the results directly, before waiting for the batch to finish. This helps reduce memory usage and reduces the rate of the request. Eg. load 100 results, process 1 by 1, load another 100 requests, process those etc.

It can still just be used in a foreach, without doing anything different.

[casperbakker]

This works like this, right?

foreach ($client->getResultsGenerator('product') as $product) {
   echo $product['idproduct'] . PHP_EOL;
}

Then I will add this to the documentation.

[barryvdh]

Yes, it only returns the product itself in the for each (no status or data object)

[casperbakker]

I have merged this and cleaned the code up a bit. Thanks @barryvdh