added "firefoxos debuggerport" gcli command to show the allocated debugger port (fix #802)
renamed gcli command to "firefoxos port"
optionally specify remote debugger port on start gcli command ("firefoxos start [port]")
FIX: port param should be optional on the firefoxos start gcli command
FEATURE: added "firefoxos listen PORT" gcli command

- if simulator is not already running, it starts it on the specified port
- if simulator is already running and it's listening on a different
  port, it request the running simulator to open a secondary listener on
  the specified port
FEATURE: gcli commands "firefoxos port get/set/reset" to manage Simulator preferred Remote Debugger port

- firefoxos port set NUMBER: set a persistent preferred simulator port
- firefoxos port get:
  - if simulator is running: print current listening port
  - if simulator is not running: print preferred port if any
- firefoxos port reset: reset the persistent preferred simulator port

when simulator is started:
- if a preferred port is set, try to use the preferred port
  - if preferred port can't be used find a new port and notify user
  - if preferred port can be used, simulator will listen on the
    preferred port
- if a preferred port is not set, silently find a free port
revert DebuggerServer.openSecondaryListener hack
fix some nits
tweaks on the remoteDebuggerPort getter
NOTE: workaround hidden simple-prefs (needed until we can use http://bugzil.la/768388)