(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
Indeed this an utterly dangerous potential bug:
```
In [1]: l = [[]] * 5

In [2]: l[0].append(3)

In [3]: l
Out[3]: [[3], [3], [3], [3], [3]]
```

Fixed.