Implement SSH authentication support in Ruby

This is a pure-ruby implementation handling the GITLAB_SHELL_KNOWN_HOSTS and GITLAB_SHELL_SSH_KEY environment variables.

Since our strategy in Ruby is to shell out to the ssh binary, we do this by writing those envvars to temporary files, then writing a script to another tempfile which is executed instead of ssh, via the GIT_SSH environment variable.

Closes #95 (closed)