Abstracting the Docker API for easier use with RSpec

See https://github.com/swipely/docker-api and https://rubygems.org/gems/docker-api for more information about the Docker API gem.

An instance of Longshoreman contains both an image object and a container object which are completely independent of each other. The image object is primarily useful for building an image from a Dockerfile, and then creating a container from that image.

If you set the DOCKER_URL environment variable before launching your tests (or script or application) the Docker API will automatically pick up the necessary information.

Create an instance with an empty container and image (the default):

l = Longshoreman.new

Create a container from a known image in `repository:tag` format and start it in
one step:

>```
l = Longshoreman.new('ubuntu:latest', 'my_container_name')

Clean up the container and/or image associated with your Longshoreman instance:

l.cleanup

Interact with the container:

>```
l.container.METHOD

Container methods include all, cleanup, create, get, id, remove, rport, start and stop.

rport is used to return the randomized port mapped by Docker:

l.container.rport(10051) => 32791

Interact with the image:

>```
l.image.METHOD

Image methods include build, remove, get, exist?, and id.

You also have full access to the Docker API gem's functionality for both the image and container instances by way of .raw:

l.container.raw.json {...json output here...}

>```
l.image.raw.json
{...json output here...}