Driver for Boot2Docker Hyper-V

Works with my custom boot2docker.iso with ident auth, see https://github.com/MSOpenTech/machine/compare/hyper-v?expand=1#diff-235ba5d4b413618fd451b3ac9da1d110R419

Requires a docker client with ident auth, as all of machine does atm.

I'm starting to merge code between this driver and virtualbox into common b2d functionality.

I'm thinking this could be built-in only for the Windows build of machine, makes sense? I can't think of a way this could be used in a Mac or Linux binary.

filepath.Join?