New protocol decoder: graycode

A new libsigrokdecode decoder that has been added a while back is graycode.

This PD allows decoding of gray code and rotary encoder protocols. Check out the source code for more details.

As usual there's a few sample files in sigrok-dumps and test-cases in the sigrok-test repository.

The decoder was contributed by Christoph Rackwitz, thanks a lot!