(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
Also for completeness, these comments are used as targets to include excerpts from example scripts in the documentation (probably not in this case I am guessing since this is a new example script).

For example here is the [html doc](http://nilearn.github.io/data_analysis/decoding.html#loading-the-data-into-python) and the [rst](https://raw.githubusercontent.com/nilearn/nilearn/master/doc/data_analysis/decoding.rst). You'll see that the comments are used as start-after and end-before targets for the literalinclude.