Release system - part 1

This is a start at implementing the requirements of #252 (with an effort of keeping pull requests small):

* Check a commit has passed Travis + Flynn CI
* Build Flynn for a given commit
* Upload built Docker images
* Build and sign a deb archive
* Add deb archive to apt repo
* Sync apt repo with S3
Any reason for the `go` subdir? I think `GOPATH=$dir` should be fine.