[MRG] 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 posted this as a WIP so that you can:
* check the testing is reasonable;
* confirm the set of methods is correct;
* contribute patches to relevant estimators.

I currently take the approach of testing for attributes only after fitting. There may be problems if ducktyping is used before fitting on, say, a `GridSearchCV`, but I guess that's just something we'll have to be aware of when working with ducktyping.

I also am not currently dealing with metaestimators that delegate their `fit` methods. (I'm not sure if they exist, but I have considered some.)

(I have pulled in the relevant commit from #1769 and removed its Pipeline-particular testing.)