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)

Closes #542.

The "git clone" on lines 37 and 41 will fail if we don't load in the proxy related environment variables for the actual system (git), not just the buildpacks.  Nothing in the cluster can see the internet, including slugbuilder, without loading http_proxy, etc.  

So in this case, to load a custom buildpack that is in fact proxy aware requires slugbuilder to also be proxy aware.

export_env_dir is just a powertool for loading environment variables; I use it with the whitelist argument to load only proxy vars. You'll also notice that  export_env_dir is the official method for loading environment variables in Heroku buildpacks.  I lifted it from the Node buildpack.  It is meant to be used with a whitelist; absolutely, don't load the whole environment.

This is the only mod to slugbuilder.  As you stated, buildpacks ultimately need to be updated to work behind a proxy.  That's what my fork does for Node:  https://github.com/MakoLabs/heroku-buildpack-nodejs/blob/master/bin/compile

For most enterprises, Internet access is locked down behind proxy gate keepers.  Nothing goes in or out without audit. Proxies are the default configuration in large enterprises that don't have breaches, LOL.