use DOCKER_HOST to connect to remote socket

The Docker client respects DOCKER_HOST to set the -H flag for the
client. Using the same envvar makes it easier for development on
platforms like OSX where DOCKER_HOST is recommended to connect to
boot2docker.

Signed-off-by: Matthew Fisher <matthewf@opdemand.com>
Merge pull request #14 from bacongobbler/dockerhost

use DOCKER_HOST to connect to remote socket