(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
You should probably use scipy.special.expit instead of defining this function: less lines of code, and faster:

<pre>
x = np.random.random(50000)
%timeit _sigmoid(x)
1000 loops, best of 3: 1.39 ms per loop
%timeit expit(x)
1000 loops, best of 3: 1.18 ms per loop
</pre>