add tests for petsc4py setup

This test will fail when the user have petsc4py installed but the default mpiexec differs from
that of the library used by PETSc. The test pass otherwise (when the user do not have petsc4py at all or when petsc4py is setup correctly).

Note:
I have used a method "which" in printing error message that resembles the "which" unix command.
However, the code of this method (the procutil.py file I've added) is not mine.. it is licenced under
the MIT licence (http://opensource.org/licenses/MIT), I've added the licence text into the file (procutil.py) but I'm not sure if it is
the right thing to do.. and I'm not sure that everybody here agree on that..