Add CNAME option

When a repo have a CNAME file, it will use the URL on it to map the server_name on ngnix
Merge pull request #562 from assaf/zero-downtime

Zero down-time deploy and server checks
Merge pull request #657 from arthurnn/CNAME_option

Add CNAME option
Merge branch 'master' of https://github.com/progrium/dokku into progrium-master
Merge branch 'progrium-master'