rename 'feature_type' to 'features throughout Menpo

In the past we supported passing strings as features - strings which would be used to decide what function to invoke on `image.features`. The name `feature_type` was used for lists of features specifications, which could be strings or actual functions.

Now we have moved to just using functions for features, this name is a little confusing and unwieldy. This PR changes the name from `feature_type` to `features` throughout Menpo.