[WIP] Ensure delegated ducktyping in MetaEstimators

This intends to solve #1805, ensuring that appropriate metaestimators have `hasattr(metaest, method) == True` iff the sub-estimator does for the set of standard methods: `inverse_transform`, `transform`, `predict`, `predict_proba`, `predict_log_proba`, `decision_function`, `score`.

I am posting this as a WIP so that you can:
* check the testing is reasonable;
* offer patches to relevant estimators 

(I have pulled in the relevant commit from #1769 and removed its Pipeline-particular testing.)