(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
Once https://github.com/nilearn/nilearn/pull/241
is merged, you should apply the _fast_smooth_array to X before learning the feature selection. It is always important to smooth a bit before applying univariate feature selection with fMRI data.