Standalone STI

Move STI to standalone repository
- Add ncdc's work to stream tars
- Refactor code and add unit tests
- Switch to using Godeps
I don't think this will work, as command ends up becoming e.g. `/opt/ruby/bin/sti-helper run`. The problem is that `sti-helper` is expecting a tar stream on stdin, which we aren't supplying for `run`.