cbft's REST api that can download cbft itself

One of the debugging/diagnostic tips from a go SF meetup was that it's useful to have the ability for clients to be able to download the server program, so that it's easy for analysis tools like pprof to work (so that your pprof dump can be matched up to the exact executable that you're running on the server).

In more detail, the tip was to add a http.FileServer, maybe export '/' or something.