Fix logfile destination for split compile/link
Report original error message in CompilationError
add mpi4py to install instructions
Add option to run compiler using os.system

On some systems (notably HPC machines) subprocess.check_call can result
in segfaults.  A somewhat safer option is to use os.system instead.  Set
the configuration option "no_fork_available" to True, or the environment
variable PYOP2_NO_FORK_AVAILABLE to 1 to enable this approach when
compiling code at runtime.
Merge pull request #409 from OP2/no-subprocess-call

* no-subprocess-call:
  Add option to run compiler using os.system
  Report original error message in CompilationError
  Fix logfile destination for split compile/link
Merge pull request #410 firedrakeproject/mpi4py-instructions

* mpi4py-instructions:
  add mpi4py to install instructions
Pull COFFEE out of PyOP2
Adapt to COFFEE's new naming convention
Update requirements and reflect them in setup.py
Update installation instructions
Update the location of the COFFEE repo