boost 1.55: Add Boost.MPI (optional)

[Boost.MPI](http://www.boost.org/doc/libs/1_55_0/doc/html/mpi.html) is a C++ interface library to the message passing interface (MPI) standard.

Please note: I do not consider this ready for merge, yet. I'm pretty sure, that this doesn't work correctly with cross-builds, and I'm not so sure if it really is the right way to add the boost.mpi library. However, since this is a fairly complex package, I was hoping for a discussion on how to do this right instead of blindly poking into it myself.