Fix MPI build for MPI library without mpi wrapper

Fix MPI build for MPI library without mpi wrapper

Some MPI library (e.g. MPICH and OpenMPI on Windows) don't
contain any MPI wrappers and thus 417d0afff broke the support for
those MPI libraries.

This change uses the FindMPI module if the compiler is not a
mpi wrapper and cmake >= 2.8.5. Requiring mpi wrappers for
cmake < 2.8.5 avoids reintroducing the problems of #851.

To avoid any confusion of different behavior with older and newer
versions of cmake, all documentation should recommend the mpi wrapper
approach, which works for all versions. The FindMPI approach should
only be discussed for advanced users (to support e.g. Windows).

As a side effect this change makes it more convinient to use with
cmake>=2.8.5. No need to speciy mpi wrapper and less problems with
nvcc.

Fixes #958

Change-Id: Ic53d8125c5a58edc6789fe16f2b710e7e2568d4f