Improve Makefile.  Add proteus/proteus_env

proteus - Launch Python with the correct environment for using Proteus
or any of its dependencies

ex: ./darwin/bin/proteus test/test_meshParitionFromTetgenFiles.py

proteus_env - Setup environment for using Proteus or any of its
dependencies

ex: source ./darwin/bin/proteus_env.sh; \
mpirun -np 4 ${PROTEUS_PYTHON} test/test_meshParitionFromTetgenFiles.py