Support for the BatchSize in retrieve requests

[fbellgr]

My very simple take on adding support for batching the retrieve requests. There was no support at all for the BatchSize option. Hoping you find this useful and satisfactory.

For clarity, you use it the same way you use the filter option:

const options = options: { filter: myFilter, batchSize: 50 }
SoapClient.retrieve(type, props, options, callback);

Regards

P.S.: You might have noticed an identical pull request before. I closed it because I had mistakenly made it from my master branch. This is the correct one.