Revert "Revert "Merge branch 'expand_envs_config' of github.com:ecnahc515/heka into ecnahc515-expand_envs_config""

This reverts commit 28a518b9f61504bf986348ac7210f9defb87774e.
Reimplement environmental variable support in config
Better comments. Remove commented code.
Merge pull request #1064 from ecnahc515/expand_env2

Second attempt at expanding env variables in config
Add dockerfiles to repo.
Update build_docker.sh and add instructions for building Docker images.
Add Docker usage example
Better emphasis on sections being files