Use MPI.jl as stand-alone module

I made some simple changes so that the shell script to run julia MPI jobs is no longer necessary. That is, you can now do ``using MPI`` without the need for the ``juliampi`` script (at least on the cluster and local machine I've tested this on).

I hard-coded the version directory (i.e. ``v0.3``) into the directory path (https://github.com/joehuchette/MPI.jl/blob/master/CMakeLists.txt#L8) which is certainly _very_ undesirable, but my Cmake-fu is severely lacking and I don't know of a way to make this generic. The build process could also be altered to remove the ``juliampi`` script entirely, if desired. Also, there's no reason the entire build process can't be automated using BinDeps; I'll take a stab at that in a bit. With that in place, it seems reasonable to package this and register it with METADATA, ref. #2.

BTW thanks for writing this package, it has been immensely helpful!