Add support for qrcode library, and make it default

Add support for qrcode library, and make it default

The qrencode library we've been using is basically dead (and has been
for a while), so add support for the newer pure-python qrcode library.

We keep support for the qrencode one around, so we don't break things
on existing installs, but consider it deprecated and should eventually
be removed.

Fixes #123