Windows file path

Since we got some issues with the path on Windows, @johngossman found the problems and PR'ed on docker (mainly the docker API) :
- https://github.com/docker/machine/issues/76 (issue on machine, on Windows)
- https://github.com/dmcgowan/docker/pull/3 (correction on the DOcker API).

I pushed the change on all path. occurences, moved to filepath, in order to well-handle all the cases wiith the "\" path separator.

It builds, i (quickly) tested on windows 64 and linux 64 inside a centos7 VM.
need to use filepath.Join for ".docker" and "hosts" here too