Fix missing mpi.eclass.

mpi.eclass is a symbolic link pointing to /var/lib/layman/science/eclass/mpi.eclass
meaning that the Gentoo Science Overlay (http://git.overlays.gentoo.org/gitweb/?p=proj/sci.git)
must be enabled through Layman for some ebuilds to work (see issue #9).
Fix issue by copying the Gentoo Science Overlay's eclass (from commit
8de3d2e7b5bca71a3d0fd216520c45f76d7146d0, see
http://git.overlays.gentoo.org/gitweb/?p=proj/sci.git;a=blob;f=eclass/mpi.eclass;hb=8de3d2e7b5bca71a3d0fd216520c45f76d7146d0).