Not shelling out for docker

TODO:

- [x] allow users to specify docker registry creds per `Bobfile`
- [x] do push commands still need to be queued like they did if we were shelling out? 
  * *turns out they don't - hooray!*
- [x] update this comment indicating which github issues this addresses
- [x] document the new stuff

This pull request addresses the following issues:

- https://github.com/modcloth/docker-builder/issues/23 - removing some `os/exec` commands (ongoing)
- https://github.com/modcloth/docker-builder/issues/83 - issue with `queued-command-runner` causing error messages to appear in the logs
- https://github.com/modcloth/docker-builder/issues/84 - make sync build wait for `docker push`
- https://github.com/modcloth/docker-builder/pull/88 - have job processing wait for asynchronous job to finish (related to #84 and #83)
- https://github.com/modcloth/docker-builder/issues/91 - use docker's `tailfile` package instead of `os/exec` to `tail -n`
- https://github.com/modcloth/docker-builder/issues/93 - server stops pushing after a while
Since we aren't executing the CLI tool anymore, it might make more sense to expose specific options via the Bobfile rather than trying to map all of the CLI flags to configuration options.