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)

@titanous  Good point.  I reused this function because it's robust a flexible, but actually we are just trying to support the git clone here.  So these are the variables required:
*http_proxy
*https_proxy
*no_proxy 
* all in lowercase and upper case form
* in this case we can pass these environment variables in from build.sh in the whitelist