Print test method names as we run them

Sometimes the DefaultDockerClientTest class can take a while to run all its
tests. This gives us some indication into progress, and lets us (and the
build system) check that the build hasn't just stalled.
Merge pull request #67 from spotify/rohan/tests

Print test method names as we run them
Add DefaultDockerClient.fromEnv

DefaultDockerClient.fromEnv creates a new DefaultDockerClient.Builder that
is prepopulated with values loaded from DOCKER_HOST and DOCKER_CERT_PATH
environment variables.

This is functionality that was duplicated and spread out among the projects
that use docker-client, so it made sense to just incorporate it into the
client itself, in one place.