(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
> The code snippet you've highlighted is essentially the code for the scoring
> logic (at least the regression case). Are you saying you want me to embed
> scoring logic into early stopping logic ?

I don't think that it is used elsewhere. So, for now, I don't see a
reason not to.