Add sample_weight parameter to metrics.log_loss

  - Also modified binary output & multiclass tests in
    test_sample_weight_invariance to test for prediction
    inputs as probabilities
  - Updated What's New
Add sample_weight parameter to metrics.log_loss

  - Also modified binary output & multiclass tests in
    test_sample_weight_invariance to test for prediction
    inputs as probabilities
  - Updated What's New
Merge branch 'log-loss' of https://github.com/jatinshah/scikit-learn into log-loss
Merging changelog notes
Refactoring - add function _weighted_sum