Move features to menpo.feature

Sticking with the theme of having few top level namespaces, this PR moves features to `image.feature`.

This is mainly due to the fact that users may go to `menpo.features` expecting that this is the main way to take the hog of an image, when of course they should be just calling `img.features.hog()`.

This keeps all the features code in one place, and our documentation will be able to reflect that.