add verifiable random buffer + add missing copyright clauses

@mortenvp I added a new random buffer class to sak that can be filled and resized anytime (and its contents can also be verified after transmission using the embedded random seed).
I thought about changing the existing random input stream class to support this functionality. I did not do it, because my understanding is that a "stream" in sak operates on a fixed-sized buffer, so it should not be dynamic. Before merging, I would like to confirm that this is the fundamental difference between a buffer and a stream.