Refactored test map/reduce for better Hadoop streaming support

Eliminated the test object and instead simply generate a confusion
matrix as the output of a reducer task. Added
merge_confusion_matrices.py to allow multiple reducer outputs to be
rolled up into a single confusion matrix. Changed display_stats.py to
take a confusion matrix rather than a test object. Updated README.md to
reflect changes.
Merge pull request #4 from bradleypallen/master

Refactored test map/reduce to better support Hadoop streaming