[ENH] ROC-based model comparison

[kaurao]

Is your feature request related to a problem? Please describe.
A way to compare models based on ROC curves.

Describe the solution you'd like
Implement the DeLong test.

Describe alternatives you've considered
Calculate it outside julearn.

Do you have a sample code that implements this outside of julearn?
https://github.com/yandexdataschool/roc_comparison

[fraimondo]

To be added to the stats module.