Add global exit selection and country code relay information

• We update our version of Arti to a custom hacky fork I just made today with experimental exit selection support, pending integrating that work into upstream.
• onion-tunnel gains a new TunnelScaffolding::locate() API for scaffolding implementations to specify which country a connection should appear from.
  • We also use the new GeoIP functionality to add country code information to relay details.
• onionmasq-mobile has a new setCountryCode() JNI API that just twiddles a variable on the scaffolding to return when the tunnel asks for a country code.
• A hacked together dropdown on the toy app lets you pick a country code to use for testing.

Closes #60 (closed). Closes #47 (closed).