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)

This method was borrowed from the official Node.js buildpack.  If you guys want to lock down the environment variables loaded (security) that functionality is there for you.  The blacklist has sensible defaults.