Proxy support

* Basically bootstraps off of the ENV_DIR support last worked on
* Allows end users to specify proxy aware custom buildpacks
* Example: https://github.com/MakoLabs/heroku-buildpack-nodejs/blob/master/bin/compile (looks for https_proxy)

I'd rather leave it up to the buildpacks to pull in the variables that they want (this is the behavior most buildpacks expect).