TST/FIX ensure correct ducktyping for metaestimators
TST extend ducktype testing to handle #2853 case
FIX ducktyping for meta-estimators
TST more rigorous testing of delegation
remove _iff_attr_has_method function
don't give names to decorators
make attribute decriptor have the right docstring.
add regression test for docstring delegation
fix docstrings in sphinx
rename decorator
Fix typos, no shortenings in name, simplify example.
Merge pull request #3982 from amueller/metaestimator_delegation

Metaestimator delegation/ducktyping. Fixes #1805.