Changed to new URL scheme with container identifier

Implementation matches design.md better:
 - http action changes from POST /add to PUT /name
 - each record now has the container identifier
Tidy up
Make error message more helpful
Make putForm more generic
Detect duplicate add
Initial Zone tests
Better error-checking
Better error-checking
Better error reporting
Better error reporting
Check duplicate add
Support DELETE function
Merge remote-tracking branch 'bb/master'
Added container updates watcher
Simplified args and added minimal event handling
Put both image builds in docker/Makefile
Moved updater into server

While we're only reacting to containers going away, it's more
convenient to bundle the docker events watcher into the server.

I've added a flag to switch it on; and switched it on by default in
the container.

I also implemented `Zone.DeleteRecordsFor(ident)`, which removes all
records for a particular container.

One minor change to the structure of the code is that the Zone is now
constructed *outside* the server, and given to it and the updater.