Add Windows detection

[kichristensen]

Detect if running on Windows, if so default to the docker namedpipe

[josegonzalez]

Registrator is meant to run within a container, in which case it's host operating system is always going to be linux based. Is that incorrect?

[jnovack]

https://docs.docker.com/docker-for-windows/faqs/#how-do-i-connect-to-the-remote-docker-engine-api

On Docker for Windows, clients can connect to the Docker Engine through a named pipe: npipe:////./pipe/docker_engine, or TCP socket at this URL: tcp://localhost:2375.

This sets DOCKER_HOST and DOCKER_CERT_PATH environment variables to the given values (for the named pipe or TCP socket, whichever you use).

[Nuschler]

host operating system can now be Windows Server or Linux
https://www.docker.com/docker-windows-server

[josegonzalez]

Thanks for the pull request!