Support remote docker hosts on OS X.

This commit brings two main changes, notably:

Two new options that can be set as environment variables

- DOCKER_OPTS: any arbitrary set of docker options. Example: --tlsverify
- DOCKER_NATIVE: This forces the use of the native docker available.
                 This is most useful if you're on OS X and do not want
                 to use boot2docker.

Now uses 'docker cp' instead of tar piping to transfer files. This
currently must be done by copying the binaries off of the docker volume
and into a local filesystem (/tmp) before a docker cp is done. This
workaround will no longer be necessary after bug fix
https://github.com/docker/docker/pull/8509 makes it into stable.

This was necessary because the tar | tar method was creating corrupted
archives on OS X even with the < /dev/null workaround.