Package build support

This adds support for an automated build process using ``Pkg.build``. I also excised the ``juliampi`` script. I still don't fully understand Cmake, but I think I hacked it enough to make it work (it builds properly and passes tests on both my Mac laptop and a Red Hat cluster, though the BinDeps chokes at the very end on the cluster, I think because of a filesystem issue).

With this, I think MPI.jl is ready for release as a package. Not sure if you're familiar with the procedure, but I can walk you through it if you'd like.