[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.