Added QR code with API key on Settings dialog
QR Code generation uses jquery.qrcode and a custom knockout binding

This way it will also be possible to easily add another qrcode to the user api key later. Refused to work for now (value binding doesn't update), so commented out right now, but works in theory.
Added changelog entry for qr code
Merge branch 'imrahil-feature_qr_code' into devel
Merge branch 'devel' of github.com:foosel/OctoPrint into devel