Make proper API by hiding uninterested methods.
Minor polishement in DRF API.
Model API now provides number of input features.
Added GBM variable importance.

- variable importance is based on formula from
Friedman paper: Greedy Function Approximation: A Gradient boosting
machine

- it is computed as empirical improvement in squared error

- by default it is reported scaled with respect to the most
important variable
Minor changes in printing variable importance.
On the return model specific variable importance object.
Fix in CM computation.

For cycle had a bad upper bound and hence omitted majority of threshold
values.
FIx in varimp scaling code.

Instead of max, min was picked.
Another fix in on-the-fly scoring and tracking AUC.

The score1 method returnes unscaled scores which were not
normalized.
Variable importance API refactoring.

- shared varimp HTML generator.
- different holders for varimp respecting computation method
- z-scores for MDA variable importance
Small UI fixes.
Merge remote-tracking branch 'origin/master' into mm_gbm_varimp