Env var configuration

This is the implementation of what I was thinking with issue #4.

To flesh out my use case some more:

If we replace /usr/bin/env on our docker image with etcdenv then we can configure it with environment variables while launching our container like this `docker run -e “ETCDENV_HOST=172.17.42.1” -e “ETCDENV_KEY=appname” imagename`.  This takes advantage of the fact that etcd on coreos is always available at 172.17.42.1. Our existing application scripts will get the environment variables because they start with `#!/usr/bin/env ruby`. This would allow us to get our configuration from etcd without any changes to our application.