(WIP) Sparse models: S-LASSO and TV-l1

- Supports TV-l1 and S-LASSO priors
- Supports logistic and squared losses
- Has cross validation
- Can automatically select alpha by CV (+ automatic computation of useful alpha ranges for the CV)
- Warning: User must supply l1_ratio
To avoid being fragile with regard to the version of scikit-learn, and also having a huge set of backports, I suggest that you copy the logic of this function into a 'predict_proba' method for your object that implements a logistic loss. That should enable you to remove all the backports.