gnunet-qr: Reimplement in C - yet only a proof of concept.

gnunet-qr: Reimplement in C - yet only a proof of concept.

Still to-do:
* running gnunet-uri
* Proper error handling
* integration into build system (automake)

Reimplementing in C was chosen since
- official zbar python-bindings support python 2 only,
- none of the other bindings available at PyPI supports the high-level
  "processor" interface which gnunet-qr uses
- implementing bindings for zbar using ctypes required addin a lot of
  low-level error handling code, thus implementing in C seamed to be
  easier,
- the programm is short, thus re-implementing is not such complicated, and
- this allows to reduce the number of dependencies (here: another
  Python version), which should ease porting to other plattforms (zbar
  is a dependency anyway).