(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
The convention is scikit-learn is that a score is always 'bigger is better'. We need to change this test_score to conform to the convention. Elsewhere it's confusing when trying to figure out what is happening in the codebase (I just went through that).