Add support for 4th extruder

#1405 includes some changes to pins.h for a 4th extruder. This commit
applies those changes. It also deals with a possible problem with
SENSITIVE_PINS.

Marlin/pins.h

 #define E1_MS1_PIN -1
 #define E1_MS2_PIN -1
 #define DIGIPOTSS_PIN -1
+#define HEATER_3_PIN -1
+#define TEMP_3_PIN -1
 
 #if MB(GEN7_CUSTOM)
   #include "pins_GEN7_CUSTOM.h"
   #error Unknown MOTHERBOARD value set in Configuration.h
 #endif
 
-//List of pins which to ignore when asked to change by gcode, 0 and 1 are RX and TX, do not mess with those!
-#define _E0_PINS E0_STEP_PIN, E0_DIR_PIN, E0_ENABLE_PIN, HEATER_0_PIN,
+// List of pins which to ignore when asked to change by gcode, 0 and 1 are RX and TX, do not mess with those!
+#define _E0_PINS E0_STEP_PIN, E0_DIR_PIN, E0_ENABLE_PIN, HEATER_0_PIN, analogInputToDigitalPin(TEMP_0_PIN),
 #if EXTRUDERS > 1
-  #define _E1_PINS E1_STEP_PIN, E1_DIR_PIN, E1_ENABLE_PIN, HEATER_1_PIN,
+  #define _E1_PINS E1_STEP_PIN, E1_DIR_PIN, E1_ENABLE_PIN, HEATER_1_PIN, analogInputToDigitalPin(TEMP_1_PIN),
 #else
   #define _E1_PINS
 #endif
 #if EXTRUDERS > 2
-  #define _E2_PINS E2_STEP_PIN, E2_DIR_PIN, E2_ENABLE_PIN, HEATER_2_PIN,
+  #define _E2_PINS E2_STEP_PIN, E2_DIR_PIN, E2_ENABLE_PIN, HEATER_2_PIN, analogInputToDigitalPin(TEMP_2_PIN),
 #else
   #define _E2_PINS
 #endif
 #if EXTRUDERS > 3
-  #define _E3_PINS E3_STEP_PIN, E3_DIR_PIN, E3_ENABLE_PIN, HEATER_3_PIN,
+  #define _E3_PINS E3_STEP_PIN, E3_DIR_PIN, E3_ENABLE_PIN, HEATER_3_PIN, analogInputToDigitalPin(TEMP_3_PIN),
 #else
   #define _E3_PINS
 #endif
   #define Z_MIN_PIN          -1
 #endif
 
-#define SENSITIVE_PINS {0, 1, X_STEP_PIN, X_DIR_PIN, X_ENABLE_PIN, X_MIN_PIN, X_MAX_PIN, Y_STEP_PIN, Y_DIR_PIN, Y_ENABLE_PIN, Y_MIN_PIN, Y_MAX_PIN, Z_STEP_PIN, Z_DIR_PIN, Z_ENABLE_PIN, Z_MIN_PIN, Z_MAX_PIN, PS_ON_PIN, \
-                        HEATER_BED_PIN, FAN_PIN,                  \
-                        _E0_PINS _E1_PINS _E2_PINS _E3_PINS           \
-                        analogInputToDigitalPin(TEMP_0_PIN), analogInputToDigitalPin(TEMP_1_PIN), analogInputToDigitalPin(TEMP_2_PIN), analogInputToDigitalPin(TEMP_3_PIN), analogInputToDigitalPin(TEMP_BED_PIN) }
+#define SENSITIVE_PINS { 0, 1, X_STEP_PIN, X_DIR_PIN, X_ENABLE_PIN, X_MIN_PIN, X_MAX_PIN, Y_STEP_PIN, Y_DIR_PIN, Y_ENABLE_PIN, Y_MIN_PIN, Y_MAX_PIN, Z_STEP_PIN, Z_DIR_PIN, Z_ENABLE_PIN, Z_MIN_PIN, Z_MAX_PIN, PS_ON_PIN, \
+                        HEATER_BED_PIN, FAN_PIN, \
+                        _E0_PINS _E1_PINS _E2_PINS _E3_PINS \
+                        analogInputToDigitalPin(TEMP_BED_PIN) \
+                       }
+
 #endif //__PINS_H