Create a relativeKeysForTree helper.
Create a relativeKeysForTree helper.

The version that exists in broccoli-kitchen-sink-helpers includes both
the original (root) directory `basename` as well as directory `mtimes`.

The issue with `mtimes` is that the `copyRecursiveSync` function
preserves them for files, but we are using `mkdirp` to create the
directories, and the `mtime`'s no longer match.

chadhietala/broccoli-caching-writer,joliss/broccoli-caching-writer,jasonmit/broccoli-caching-writer,ember-cli/broccoli-caching-writer,stefanpenner/broccoli-caching-writer,twokul/broccoli-caching-writer