Added support for docker-api gem as client, removes requirement that
cli tools be installed on host running test-kitchen. This also added a
few additional options:

:ruby_client -- (true/false) select whether to use the docker-api as
  your docker client (default false)

:read_timeout -- (int) Choose read timeout used by docker-api during
API options. This originally defaulted to 60 seconds which was too low
for some Dockerfile operations such as large RPM installs on the first
(uncached run), bumped to a workable default and made configurable
through kitchen.yml should someone need it to be higher
Updated error message if validation fails using docker-api
Parameterized Cmd to support the run_command override
Removed all depenendies on CLI client & removed ruby_client: option
Merge pull request #27 from adnichols/docker-ruby-api

Added support for docker-api gem as client