APT repository for RabbitMQ

Previously we've attempted to run a repository in the same way that 
Debian would: have repository management software installed on the 
server, and upload new packages to the repository as and when they're 
ready.

This turned out to be both fiddly and annoying to do (and more 
particularly to automate) so since our repository is always going to be 
small it's easier just to create the entire repository as part of the 
build process, just like a package. It can then be moved into place as a 
single unit. The make target "debian_apt_repository" (invoked by "dist") 
will create it, and it can get moved onto the server with the rest of 
the packages.

Read "README-real-repository" for information on how we used to do 
this.
