Fusion of buildpack-multi with overlay

Summary:

Since the kickoff of the heroku app of the intel team needed to use custom buildpacks, a fusion of the buildpack-multi and overlay was needed.
The strong point is that any other team who needs custom buildpacks can migrate to this buildpack using the instrucions on the README

Test Plan:

 - running ./bin/detect must always echo a simple string with exit 0
 - running "./bin/compile ." (local) will exit with error if the local environment $OVERLAY is not set.
 - running "./bin/compile ." (local) will exit with error if there is no .buildpacks or .buildpacks.$OVERLAY file
 - set overlay to baz (export OVERLAY=baz), touch foo.bar.baz, and create a .buildpacks. (echo https://github.com/heroku/heroku-buildpack-python > .buildpacks) then run "./bin/compile .". foo.bar.baz must be overlay to foo.bar and it must start downloading the buildpack

CCs: