Currently, the metrics.py module is becoming huge and contains 2345 lines. It becomes difficult to enter in the module to newcomers. I propose to split sklearn.metrics.metrics
in

• sklearn.metrics.classification which would contains all classification metrics without score involved
• sklearn.metrics.regression which would contains all regression metrics
• sklearn.metrics.ranking or sklearn.metrics.threshold which would contains all classification metrics with a score involved.

The sklearn/metrics/tests/test_metrics.py, which contains 2710 lines, would be split in 4 testing files:

• sklearn/metrics/tests/test_invariance.py with invariance tests and common tests to all metrics
• sklearn/metrics/tests/test_classification.py
• sklearn/metrics/tests/test_regression.py
• sklearn/metrics/tests/test_ranking.py or sklearn/metrics/tests/test_threshold.py

What do you think?