(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
> For the haxby epi mask, cropping screens-away upto 37% of the original box.

And that's pretty sweet: factor of 3 improvement in the proximal
operator!