receiver, slugbuilder: Support ENV_DIR.

The git archive gets rewritten to contain a cookie file .ENV_DIR and a two
directory structure (/app and /env). Slugbuilder now detects those slugs and
acts accordingly, while still allowing just plain ol' git archive slugs.

Closes #97.
I thought so, but [some buildpacks](https://github.com/heroku/heroku-buildpack-python/blob/master/bin/compile#L64-L66) seem to expect a specific location.