[MRG] FIX: Bug in RFECV when step is not 1

Hi!

This patch fixes the bug with computation of the final `n_features_to_select` when `step` is not equal to 1. Everything should be clear from the code.

Please review.
So if step >1, grid_scores_ has length ``X.shape[0] // step + 1``, otherwise ``X.shape[0]`` ?