[MRG+1] Return attribute n_iter_ for linear models dependent on the enet solver

This should be quick review and will help to finish or close the other pull requests. @agramfort @ogrisel please have a quick look.
I see the reasoning behind `n_iters` instead of `n_iter` here but I still find it confusing.  I suggest `n_iter` as well. What do you think @ogrisel?