Support the ENV variables used by boot2docker.

Boot2docker currently installs docker 1.3, which uses https and certificates. It sets up the `DOCKER_HOST` and `DOCKER_CERT_PATH` environment variables. The docker client understands these variables, so it would be nice if docker-api understood them too.

This should fix #202 without the need for custom configuration.

I did my best to minimize changes and to get spec coverage for the code change. I would be happy to incorporate any feedback into the diff.

Please consider merging this, so docker-api can work with boot2docker on OSX out of the box! :heart: 
Thank you for catching that!

Yup, I assumed that the test case is taking advantage of some RSpec shorthand.

I fixed this pattern everywhere in the spec for the Docker object.

Also, I had to copy-paste `Docker.options = nil` to every before block that changes environment vars, because the docker's before block executes before the context-specific before blocks, so options never actually changed before.

Previous tests didn't catch this because they were all testing for the same result, namely the empty hash.