Dogfood landmark_resolver in menpo.io

`landmark_resolver` is currently a special case which leads to duplication of code in the `_import` method. Not only is this inelegant, but it actually causes us some headaches in dealing with the upcoming removal of the group label from a `LandmarkGroup`. This dogfoods `landmark_resolver` with a default resolver for Menpo, that behaves exactly as it does in master now anyway. This also clears the way for the removal of label from `LandmarkGroup`.