(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, I've tried my best to build an comprehensive authors' list.

@everybody: Please let me know if you think you've been forgotten, or your code has been plagiarized! Better still, simply go ahead and (dis-)claim ownership by deleting / appending / prepending your name from / to the authors' list concerned module / script. This is an issue like any other code bug, and you shouldn't hesitate to provide fixes, etc.

I'm done with this for now.