Get basic artifacts movement working (this has a number of issues to discuss!)

Issues:

- Right now, the "outbox" is .winchman/out and the "inbox" is .winchman/in/{previous-unit-id}. These are currently hardcoded, but we should make them configurable (requires a change to the graph-builder)
- Because of the hard-coded nature of above, Dockerfiles have to create the ".winchman/out" directory themselves.
- This depends on a change to the builder-core to not delete temporary images (PR #20)
- This depends on a change to the builder-core to always log the image_id (PR #21)
- The copying of the log code to pull out the image_id is ugly. This code is temporary, and we need to figure out a nicer way.
- appendBuildArtifacts has to currently move the 'out' directory. Artifactory should be updated to allow us to specify a prefix to be removed
This functionality has now been merged into builder-core ([link](https://github.com/winchman/builder-core/blob/master/communication/log.go#L27-L44))