docker_compose-rs: Support for working with docker-compose.yml files

This is a work in progress! I'm still implementing the basic file format.

You can build this library using stable Rust version 1.11. But if you want to develop it, you will get much better error messages using a nightly build of Rust.

```sh

Install Rust stable and nightly using rustup.

curl -sSf https://static.rust-lang.org/rustup.sh | sh rustup toolchain install nightly

Build unit tests using nightly Rust.

rustup run nightly cargo test --no-default-features --features unstable ```