Added CHDK support

Added so M240 can be used to trigger CHDK instead of sending an IR signal, see more about CHDK here http://captain-slow.dk/2014/03/09/3d-printing-timelapses/

Marlin/Configuration_adv.h

 //=============================Additional Features===========================
 //===========================================================================
 
+//#define CHDK 4        //Pin for triggering CHDK to take a picture see how to use it here http://captain-slow.dk/2014/03/09/3d-printing-timelapses/
+#define CHDK_DELAY 50 //How long in ms the pin should stay HIGH before going LOW again
+
 #define SD_FINISHED_STEPPERRELEASE true  //if sd support and the file is finished: disable steppers?
 #define SD_FINISHED_RELEASECOMMAND "M84 X Y Z E" // You might want to keep the z enabled so your bed stays in place.