Support running Flintrock from Windows

[nchammas]

This is an umbrella issue for all the things that need to be done to support running Flintrock on Windows:

• Remove dependency on ssh-keygen.
• Remove dependency on ssh for interactive login.
• Get AppVeyor build setup.

[nchammas]

I likely won't be able to do this myself, but I am willing to help a motivated contributor work through all the things required to get Flintrock working on Windows, as long as the necessary changes seem reasonable and do not impose too much of a maintenance burden.

[nchammas]

I'm going to close this issue for now since it doesn't have any momentum, but I stand by my earlier offer to help a motivated contributor add this support.