Transparent barcodes

[NigelThorne]

I've been hacking a new renderer for your library to make it possible to have see though barcodes.

I wrote a version of the QRCode class to do this. If you are keen I'll raise a pull request, and you guys can make it production ready.
Let me know if you are keen.

[codebude]

Merged it. Thank you very much - great piece of work!