ENH print number of fits in BaseSearchCV._fit

As mentioned on the ML (@robertlayton), this makes the grid search a little bit friendlier by warning you how big your grids are!

It might be nice to have a method that returns the fit count in advance, but it doesn't fit with the current class model and I don't have the time to change that now.

It is also only smoke-tested (and the case where `parameter_iterator` does not have `__len__` is not tested).