(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
@GaelVaroquaux it is unclear where the priority is: this particular portion of the PR looks like a new feature and may actually slow down the process. Should it be kept for a future PR ?