[MRG] FIX: more robust skip of implicit constructor

Parameter introspection requires introspecting the keyword arguments of the
constructor of Python classes deriving from BaseEstimator.

This can be problematic for classes that do not override the default
constructor. Introspecting the implicit constructor used to raise a TypeError
but this is no longer the case in Python 3.4+. Hence with use a explicit check
for the implicit constructor prior to using inspection.