Added QR codes in pastes

Added global support for QR codes.
These QR codes are generated with phpqrcode (http://phpqrcode.sourceforge.net/).

QR codes are cached as PNG images in static/qr/

Generation of a QR code is made:
- when a paste is created
- when a paste without a previously generated QR code is viewed

On paste delete, the corresponding cached PNG is also deleted

This feature is displayed in themes "default" and "stikkedizr" with this commit.