[FEATURE] Run processes in one process or separately.

[FEATURE] Extract GatewayProcess class for DRYness.

[FEATURE] Optionally support bin/gateway start --stack.

[FIX] Remove `gatewayd` require.

[FIX] Affix gatewayd.ripple.Listener.

[FIX] Typo in processes/server.js

[FEATURE] Defaut start with single process, with pm2 option.