Issue-21 - refactor to use docker-api gem

This is a first swipe at refactoring centurion to use the docker-api gem. There are a few caveats I would like to point out.

* the list action seems broken (even in the current version) so I left that code as is. If you feel that task is really important then we should determine how to handle that

* Attach is rather difficult to achieve in the docker-api gem. They have support but I could not find the right incantation to log in to a running container. I asked in the issue if you guys still have a usecase for this, if so we can work through adding the right support.

* Because you guys used the cli for issuing the docker pull it took advantage of the `.dockercfg` file being on disk for the credentials. I added a registry_username and registry_password to the dsl and as cli arguments.

** The custom environments commit was on this branch so I removed it ** 

I am going to keep working on this branch but just wanted your opinion which is why I am submitting now.