(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
> You're right we don't need this anymore (this is residual hack from the
> original plot_oasis_vbm.py script). Will cleanup asap.

Awesome! It's always a pleasure to see hacks disappear from a codebase.