[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.
Hum, but the RFE fits only 5 estimators, right? Sorry I haven't looked at this code in a while.