Build Matrix for Serial and Parallel Version

- Enable building libSplash without any MPI support at all
- Add WITH_MPI Flag for example/ (same option -DWITH_MPI=ON/OFF)
- include <mpi.h> only in examples itself
  (note: always include mpi.h BEFORE c-stdlibs due to conflicting
         defines in some c-libs)
Relax complexity of run_parallel tests script
Huuups, testing MPIRUN works different
Add WITH_MPI flag for tests

- enables a subset of tests to perform a plain "serial, non-MPI test"
Remove doubled entries in run_parallel_tests
Add missing MPI_Init()'s for some tests
Merge pull request #12 from ax3l/topic/travis-buildmatrix

Build Matrix for Serial and Parallel Version