Validate the SSH public key file before attempting to add it.
Refactor Git commands into a new GitHelper.
Add some functional tests for GitHelper.
Cleanup
Allow toolstacks to write to $output and use shell abstractions.
Merge pull request #174 from pjcdawkins/validate-ssh-key

Validate the SSH public key file
Make toolstack detection error less scary - addresses #173
Fix double error output from shell commands.
Merge pull request #167 from pjcdawkins/git-helper

Refactor Git/shell/Drush commands; and let toolstacks write output
Update tests now that ShellHelper throws a generic exception.
Add init command.