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.
Use `cmd.StdinPipe` and run `appendEnvDir` in a goroutine so that we don't have to buffer the entire tarball in-memory.