Add Ruby 2.0.0 as known to work in README.
t2-delete-runs: Minor code clean-up.
t2-delete-runs: Add a flag to only delete finished runs.

Can be combined with the --all flag to delete all finished runs.
t2-run-workflow: Minor code clean-up.
t2-run-workflow: Add flag for service credentials.

This allows the user to specify credentials for any remote services that require
them, e.g. R servers.