Support for non-git buildpacks

Currently `BUILDPACK_URL` [has to be a git repo](https://github.com/deis/slugbuilder/blob/e500449f8b107d91f35326db70e5ffb20162c0f7/rootfs/builder/build.sh#L126).

There is ([at least one](https://github.com/heroku/heroku-buildpack-emberjs)) a buildpack which is distributed in pre-complied form (*.tgz). It's possible to pass link to the archive in heroku, buildpack in git requires some manual action (complication), so it won't work in Workflow.

Is it possible to re-implement this in Workflow?

As workaround I've unpacked provided archive and pushed it to separate git repo.