Fixed #29 -- Generate the QR code locally

I'm doing this for two reasons:
 1. I don't want Google seeing the secret keys to all my tokens.
 2. The Google QR code chart is deprecated. See: https://developers.google.com/chart/infographics/
Prepare for upcoming 0.5.0 release
Allow overriding the QR Image factory.

This way if PIL/pillow is available PNG images can be used, etc.