Estimators don't have n_iter_ parameter

Scikit-learn estimators have a `n_iter_` parameter. Dask-GLM doesn't.

For example, the Scikit-Learn logistic regression implementation has the `n_iter_` parameter ([docs][sklearn]). However, the Dask-GLM implementation doesn't ([docs][dask]).

This issue presented itself in https://github.com/dask/dask-blog/pull/15#discussion_r264820847

[sklearn]:https://scikit-learn.org/stable/modules/generated/sklearn.linear_model.LogisticRegression.html
[dask]:http://ml.dask.org/modules/generated/dask_ml.linear_model.LogisticRegression.html