Require --mode argument. Don't guess from config file

* Slave/Standalone: Look for opts.port, not opts.listen. Matches what the
  config file has always used.
* Streamer: Reshape mode handling
* Update nconf requirement
* Update mincer requirement
Standardize handoff handling on USR2

* Mode.Base: Add constructor to attach SIGUSR2 handler.  Standardized handler
  looks at @opts.handoff_type to determine whether we're spawning our own
  new process ("internal") or relying on an external program like lifeguard
  to do that for us ("external", default).  Once it gets HANDOFF_GO, it starts
  up a translator, then looks for `streams`. Once found, it calls
  `_sendHandoff`
* All modes now call `super` in their constructor and use `@opts` instead
  of `opts`
* Standalone: Update handoff sequence to match other modes