Add hello --> world example

The example prints a GPU buffer "hello", then receives a *char from a CPU process and prints the content (world).

I think it's a good intermediate step between 01 and the previous 02. What do you think, @achilles4?

The code is not very elegant and just does this one thing. And it only works in the one configuration (CPU rank 0, GPU rank 1), as I wanted to leave the GPU code free of the word world!.