(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
OK, it's sad to see *Lisp* end this way. I'll hereby promise to make a tremendous effort to mitigate my use of the *Lambda* in the codebase henceforth :)