Merge pull request #7261 from betatim/negative-scorers

doc/modules/model_evaluation.rst

examples/model_selection/plot_underfitting_overfitting.py

examples/plot_kernel_ridge_regression.py

sklearn/linear_model/tests/test_ridge.py

sklearn/metrics/scorer.py

sklearn/metrics/tests/test_regression.py

sklearn/metrics/tests/test_score_objects.py

sklearn/model_selection/tests/test_validation.py

sklearn/tests/test_cross_validation.py