Add ENV['DOCKER_HOST'] as default socket and add docs
added a stderr redirect to the docker binary check because docker on OSX uses stderr to print output and ruby is interpretting this as an error
set use_sudo flag to false when using the run command to check if docker exists
tell verify_dependencies to be quieter

Running anything `kitchen`-related with kitchen-docker now produces lengthy stderr output, which makes it harder to see things.

Add redirection for STDERR when running the docker binary, as now running the `docker` command outputs to STDERR (version 1.2.0).
This is addressed in a [not-yet-released commit](https://github.com/docker/docker/commit/61b129d81802e3c988cc0e67e488b24968dd748a) where the `help` command is redirected.
added gentoo support
enable ssh PasswordAuthentication

on gentoo PasswordAuthentication is disabled in the default
configuration for security reasons, we have to enable it.
added emerge and paludis support for gentoo
added missing sudo for gentoo
add check for valid container id
update dns flag, make inspect command quieter
Merge branch 'master' of https://github.com/peterabbott/kitchen-docker