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: