Add -host-ip argument for cases when the hostname resolves to docker0

The containerized version of registrator sets the IP address of the service as the hostname, which resolves to the docker0 address, and not the externally visible address. Added -host-ip to allow that to be fixed. 

Also added a log message to indicate the backend + uri, since I kept setting it wrong and it would hang because the uri was wrong. :) 
Since we assume this *will* be run in a container, really we want the user to pass the host's hostname as the container hostname, basically as a requirement. So the user shouldn't ever run into it resolving to the docker0 bridge. The first strategy when you want registrator to use a special IP is by telling Docker to use a specific IP when publishing the port. The `-ip` argument should really just be for edge cases...