Add to the "system:" section of config.yaml

There are two types of event handlers at the moment: 
	system: invokes an external command without waiting for the result
	gcode: sends some gcode to the printer.  Separate multiple commands with a comma

Example: 
		system: 
		  events:
		  - event: Connected
			type: gcode
			command: M115,M17 printer connected!,G28
		  - event: Disconnected
			type: system
			command: python ~/growl.py -t mygrowlserver -d "Lost connection to printer" -a OctoPrint -i http://rasppi:8080/Octoprint_logo.png
		  - event: PrintStarted
			type: system 
			command: python ~/growl.py -t mygrowlserver -d "Starting _FILE_" -a OctoPrint -i http://rasppi:8080/Octoprint_logo.png
		  - event: PrintDone
			type: system 
			command: python ~/growl.py -t mygrowlserver -d "Completed _FILE_" -a OctoPrint -i http://rasppi:8080/Octoprint_logo.png

command values support the following dynamic tokens:
		_DATA_ 		- the data associated with the event (not all events have data, when they do, it's often a filename)
		_FILE_ 		- filename of the current print (not always the same as _DATA_ filename)
		_LINE_ 		- the current GCode line
		_PROGRESS_ 	- the percent complete
		_ZHEIGHT_ 	- the current Z position of the head
		_NOW_		- the date and time of the event
		
			
Available Events:

  Startup		-- the server has started
  Connected		-- the server has connected to the printer (data is port and baudrate)
  Disconnected	-- the server has disconnected from the printer
  ClientOpen	-- a client has connected to the web server 
  ClientClosed	-- a client has disconnected from the web server
  PowerOn		-- the GCode has turned on the printer power via M80
  PowerOff		-- the GCode has turned on the printer power via M81
  Upload		-- a gcode file upload has been uploaded (data is filename)
  LoadStart	    -- a gcode file load has started  (data is filename)
  LoadDone      -- a gcode file load has finished  (data is filename)
  PrintStarted	-- a print has started (data is filename)
  PrintFailed	-- a print failed (data is filename)
  PrintDone		-- a print completed successfully (data is filename)
  Cancelled		-- the print has been cancelled via the cancel button (data is filename)
  Home			-- the head has gone home via G28
  ZChange		-- the printer's Z-Height has changed (new layer)
  Paused		-- the print has been paused
  Waiting		-- the print is paused due to a gcode wait command
  Cooling		-- the GCode has enabled the platform cooler via M245
  Alert			-- the GCode has issued a user alert (beep) via M300
  Conveyor		-- the GCode has enabled the conveyor belt via M240
  Eject			-- the GCode has enabled the part ejector via M40
  CaptureStart	-- a timelapse image is starting to be captured  (data is image filename)
  CaptureDone	-- a timelapse image has completed being captured (data is image filename)
  MovieDone		-- the timelapse movie is completed (data is movie filename)
  EStop			-- the GCode has issued a panic stop via M112
  Error			-- an error has occurred (data is error string)