Rename CDNServeMux to CDNBackendServer

This was never technically a ServeMux because it sends all requests,
regardless of path, to the same handlerfunc. Although it implements
http.Handler I don't think it's a "handler" either because it also contains
references to the underlying server. So CDNBackendServer seems like the most
appropriate name for it.
Merge pull request #55 from alphagov/rename_cdnservemux

Rename CDNServeMux to CDNBackendServer