Pkg.build support

Adds support for building MPI.jl using the ``Pkg.build`` command, among a few other small refactoring changes (mostly changing the directory structure). Closing #6 in lieu of this one, which works properly on the three machines I've tested it on. Still have the version directory hard-coded, but I'm not sure of a way around that.

Edit: just did battle with git, and everything is nice and squashed now.