Allow environment variables in the configuration file

This eases deployment by allowing environment variables in private_pub.yml.

Because Faye needs to run in production mode, the yaml file's `production` block gets used for both development and production, making private_pub hard to deploy.

    production:
      server: <%= ENV["FAYE_SERVER"] %>
      secret_token: <%= ENV["FAYE_KEY"] %>

This change also forces signature_expiration to an integer regardless of how it is entered in the configuration, and it adds tests.