(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
Yeah, i have other backward-compat problems with center_data (not present sklearn <= 0.10). I'm implementing some lightweight backports, so that travis can let me go...