Bootstrapping importing update - 3rd times a charm

- External bundle registration no longer generates a task and ZincImportExternalBundleTask has been removed
- The files of an external bundle are now tracked by the ZincRepo. If a file is needed by a bundle clone, it will first try to copy it from the local, external source.
Removing dead test
Adding missing file
Safer relative path calculation
Force bundle reregistration if files don't exist
Moving localFilesBySHA initialization to init method
Merge pull request #47 from mindsnacks/feature/external-bundles-take-3

External bundles take 3