Added .pid files to ipcluster and ipcontroller and daemon mode....
Added .pid files to ipcluster and ipcontroller and daemon mode.
* The ClusterDir object now creates a /pid sub-dir for storing .pid
files.
* Both ipcluster and ipcontroller now create .pid files upon starting.
* Both ipcluster and ipcontroller check for the existance of .pid files
before starting and won't start if another instance is running.
* ipcluster has a daemon mode (--daemon/--nodaemon) will daemonize
the process and then write the .pid file.
* Added a "ipcluster stop" subcommand which looks for a .pid file
and kills the process.
