py.test fails if DPCOMP_LOG_PATH is not set

[simsong]

Running py.test without setting DPCOMP_LOG_PATH generates this error:

_______________________________________________ ERROR collecting test/mechanism/test_dataset.py ________________________________________________
/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/py/_path/local.py:662: in pyimport
    __import__(modname)
test/__init__.py:5: in <module>
    LOG_PATH = os.environ['DPCOMP_LOG_PATH']
/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/UserDict.py:40: in __getitem__
    raise KeyError(key)
E   KeyError: 'DPCOMP_LOG_PATH'
____________________________________________ ERROR collecting test/mechanism/test_query_nd_union.py ____________________________________________

These environment variables are referenced in the README.md, but the failing could be more graceful than the failure experienced above.