[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.
``n_features_to_select_by_rank == [1, step, 2*step, ..., X.shape[0]]`` right?