Serve BärCODE in other formats.
This MR makes it possible to serve BärCODEs in different formats. This is done via the "Accept" header in the HTTP request. Depending on the mime-type set in this header the format of the BärCODE will change:
- image/png: Raw PNG of the QR code.
- image/png;encoding=base64: Base64 encoded PNG of the QR code.
- application/baercode: Raw BärCODE data.
- application/baercode;encoding=base64: Base64 encoded BärCODE data.
This closes #8.