Include mpi.h in base/config.h to avoid Intel compile error

On my system with Intel compiler, I used to get the following error:
In file included from /lrz/sys/intel/mpi_41_3_048/intel64/include/mpi.h(1279),
                 from [...]/deal.II/include/deal.II/base/mpi.h(23),
                 from [...]/deal.II/include/deal.II/base/utilities.h(21),
                 from [...]/deal.II/include/deal.II/base/index_set.h(20),
                 from [...]/deal.II/include/deal.II/lac/vector.h(24),
                 from [...]/deal.II/source/numerics/data_out_faces.cc(18):
/lrz/sys/intel/mpi_41_3_048/intel64/include/mpicxx.h(99): error: #error directive: "SEEK_CUR is #defined but must not be for the C++ binding of MPI. Include mpi.h before stdio.h"
  #error "SEEK_CUR is #defined but must not be for the C++ binding of MPI. Include mpi.h before stdio.h"
   ^

This is Intel compiler 15.0 with Intel MPI. The solution to this problem is to include mpi.h early enough, which for us means to include it in base/config.h before we include types.h.
Merge pull request #271 from kronbichler/master

Include mpi.h in base/config.h to avoid Intel compile error