Added helm faces to the Faces customize group.

Previously the only way to access the customization options for
the helm faces was through Convenience > Helm and they were there
in the top level view. Now, the customization options are
accessible through the subgroup in Helm > Helm Faces, or through
Faces > Helm Faces. The customizations of the subgroups, such as
helm-buffers, follows the same convention -- accessible both
through Helm > Helm Buffers > Helm Buffers Faces and Faces > Helm
Faces > Helm Buffers Faces.

When trying to customize helm it took me a while to find the location of 
the helm faces customizations. This pull request attempts to make the
discovery of the helm faces customizations a little more intuitive.