(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
This is great, bit for the sake of future maintenance, please indicate whether each feature put in this module has been introduced in skl (say) 0.14 or 0.15, so that this part can be safely removed within a few years.