Docker (CentOS 7)

### New Feature

- Includes @drewsonne's PR #15 to bring Docker support (with CentOS 7 + ruby 2.0.0p353) - thanks Drew!
- Modifications of `Dockerfile` to increase use of docker cache
- Included `.dockerignore` to reduce size of container
- Used `ENTRYPOINT` to remove need for separate executable

- Still to do: remove development libraries & clean up after bundle install.