WIP: Added Docker-machine support for Windows/Mac

An improved version of https://github.com/inducer/relate/pull/212

1. Support relate docker utility on a local Windows docker-machine, not tested on Mac (no access for) though.
2. Check dependencies of docker-machine programming, on whether installed docker and docker-machine, running status (if not running, startup docker-machine) and whether env variables have been exported (if not, print the export script for user to execute).
3. Put all docker (docker-machine) configurations into RELATE_DOCKER_CONFIGURE, except RELATE_DOCKER_ENABLED.
4. Validate RELATE_DOCKER_CONFIGURE at start up, by trying to creating a docker Client instance.

• Another MR, which add more checks for potential problems which currently cannot be detected at start up.
• Backward compatibility of settings.
• Documentation