Add qty property to PrintJob #5

I've been advised by support@printnode.com to use the qty property instead of copies of print options to set number of copies when sending raw zpl jobs. This seems to be missing from the PrintNode-Python library. In my experience, using copies works for raw jobs on windows and mac clients but does not work on linux.

https://www.printnode.com/en/docs/api/curl#printjob-creating

Currently using this

gateway.PrintJob(
       printer=printer_id, options={"copies": num_copies}, job_type="raw", base64=base64_raw_bytes_decoded
    )

From printnode api documentation, one would expect the following to work

gateway.PrintJob(
        printer=printer_id, qty=num_copies, job_type="raw", base64=base64_raw_bytes_decoded
    )