CV Attributes

Partially completed #2709. I'm opening this pull request to get feedback on the implementation of best_*_ attributes for LassoCV and ElasticNetCV and ideas for how to test this further. I noticed some inconsistencies between LassoCV/ElasticNetCV and GridCV with Lasso/ElasticNet which makes it difficult to validate against GridCV.

I'll continue with RidgeCV, LassoLarsCV, etc assuming this looks good so far.