make DirectoryLister use HtmlResource to honor 'view' authorization property

I've never before used buildbot, but in my initial setup today I made it such that slaves upload their build artifacts to the master's `public_html/builds` directory.

Our buildbot instance is on a public server, so we wanted to have this directory behind some sort of an authentication layer. After updating to the dev build, I noticed the 'view' property, and it _seemed_ to make sense that the directory listings would also honor this.