Zero down-time deploy and server checks

This change makes Dokku start up the new container, run a set of checks
against it, and only switch traffic over to the new containers if all
checks complete successfully. No requests are dropped during the switch
over.

To specify checks, add a CHECKS file to the root of your project
directory. This is a text file with one line per check. Empty lines and
lines starting with # are ignored.

A check is a relative URL and may be followed by expected content from
the page, for example:

  /about     Our Amazing Team

Even if you don’t use any checks, this change will prevent downtime
during switching from old to new container.

See: https://labnotes.org/zero-downtime-deploy-with-dokku/
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 awaiting-prs