Added environment support to the install buildpack process

- If a custom buildpack is being used chances are it's because there is something unique about the environment
- The most standard annoyance in enterprise environments is the lack of direct outbound connectivity
- Proxy support needs to be propogated during the entire end to end build process.
- http_proxy and https_proxy can be added as environment variables
- Custom buildpacks can use the environment variables to be proxy aware
- install-buildpack is now proxy aware; it simply loads the environment variables from ENV_DIR
Added ENV_DIR and Proxy support

git will pick up https_proxy and http_proxy if set and use those for the clone
Removed comment
Fixed variable escaping for $env_var
Restrict environment variables loaded

- Restrict to just proxt related environment variables
- Support upper and lowercase form
Removed whitespace
Merge pull request #546 from tlperkins/master

Proxy support