[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.
Ok it is a bit more complex again :-/ It would be nice to write down how long ``grid_scores_`` is but it seems non-obvious to me. Have you tried writing it down?