Add commas to list of extensions

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Correctly document new auth environment variables

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Rename HTTPS docs to authentication

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
add redirect for https docs to authentication

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Use path/filepath instead of path (for Windows)

the code is using path.Split, path.Join etc. instead of filepath.Split, filepath.Join. path assumes '/' separators, but Windows of course uses '\'. Talk about the gift that keeps on giving...I've been fixing bugs like this in portable code since 1983.

See https://github.com/docker/machine/issues/76

Signed-off-by: John Gossman <johngos@microsoft.com>