(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
scipy's expit is buggy (not well coded) and can encounter overflows. Here: http://stackoverflow.com/questions/22006650/scipy-expit-unexpected-behavour-nans