Improvements of the RFE module.

This commit includes the following list of changes:
- Documentation has been enhanced and completed.
- Examples have been added.
- The `percentage` (float) parameter has become `step` (int or float), and indicates the number of features to remove at each iteration (int), or the percentage of features to remove (float) with respect to the original number of features.
- Exactly `n_features_to_select` are now always selected. It may not always have been the case before, as too many features could have been removed at a time in the last step of the elimination.
- The `ranking_` attribute is now a proper ranking of the features (i.e., best features are ranked #1).
- The code of `RFECV` has been made simpler.
- The `cv` argument of RFECV.fit has been moved into the constructor and is now passed through `check_cv`.
- Tests.