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.