cmake: introducing automatic MPI selection

This adds a new CMake parameter `pfasst_WITH_MPI` (default OFF) to
enable automatic discovering and use of MPI compiler.
When a MPI compiler has been found, MPI depended examples (i.e.
advection_diffusion/mpi_pfasst) get built.

contributes to #32
travis: another try on enabling MPI on Travis
cmake: add make targets to run compiled examples

also collective targets to run all examples; travis will run those
Merge pull request #2 from torbjoernk/feature/mpi-pf

try fixing Travis and improve MPI+CMake