(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
There is something that makes me uneasy with the code here. You are grabbing huge portions of the codebase of scikit-learn, and slightly modifying them. There must be better ways to do things.