Add tooling & support for running tests in Docker.

This pull request is a result of a convo in #108.

## Docker

On my host machine I run Emacs 24.4. However, the latest stable release of Emacs is 24.3. We are in a situation where `elixir-mode` is using features of SMIE only available in 24.4+. As a solution, @sviridov said he uses a Docker with Emacs 24.3 installed, and runs tests against that. This sounded like a great idea so I did my best to implement such a thing.

The goal here is that contributors will interact with Docker only through `rake` tasks, which I'll go over next.

## Rake

This PR adds four new `rake` tasks:

- `docker:install:ubuntu:latest` - Installs the latest version of Docker for Ubuntu host systems.

- `docker:install:ubuntu:stable` - Installs the latest OS/stable release of Docker for Ubuntu host systems.

- `docker:build` - Builds an image from the `Dockerfile`.

- `docker:test` - Runs the tests in Docker.

There is a known issue with running the tests on Docker. So things appear to work fine on the initial pass of  `./run_tests`. However, when the tests fail, and I press `Y` (or just hit `<RET>`) to run the tests in emacs, I get a whole mess of failures. I think there is probably some configuration of the `.emacs` file in Docker that needs to be done. It seems, for now, to be beyond the scope of this here.

@sviridov I would love your input on this so it can be better.