(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
I suggest that we drop the 'CV' names. The neuroimaging end user will not understand what CV means, and we'll just say in the docstring that these objects set their parameters by cross-validation.