added mpi_hello.dats

Added everything except proper compilation (I was at `PATSCCOMP="mpicc -std=c99 -D_XOPEN_SOURCE" patscc mpi_hello.dats` before my time ran out) and handling of hostname in the example. I figured you might have better ideas for string handling than I could easily come up with. 

Also, I probably won't be able to work on this much over the next week, but maybe a bit here and there.