Implement M226 - GCode Initiated Pause

Implemented M226 as described here:
http://reprap.org/wiki/G-code#M226:_Gcode_Initiated_Pause

Waits for pin to be become either HIGH, LOW or the inverse of what it
was before. Allows printing to pause until user interaction

Marlin/Marlin_main.cpp

@@ -139,6 +139,7 @@
 // M218 - set hotend offset (in mm): T<extruder_number> X<offset_on_X> Y<offset_on_Y>
 // M220 S<factor in percent>- set speed factor override percentage
 // M221 S<factor in percent>- set extrude factor override percentage
+// M226 P<pin number> S<pin state>- Wait until the specified pin reaches the state required
 // M240 - Trigger a camera to take a photograph
 // M250 - Set LCD contrast C<contrast value> (value 0..63)
 // M280 - set servo position absolute. P: servo index, S: angle or microseconds
@@ -2147,6 +2148,57 @@ void process_commands()
       }
     }
     break;
+
+	case 226: // M226 P<pin number> S<pin state>- Wait until the specified pin reaches the state required
+	{
+      if(code_seen('P')){
+        int pin_number = code_value(); // pin number
+        int pin_state = -1; // required pin state - default is inverted
+
+        if(code_seen('S')) pin_state = code_value(); // required pin state
+
+        if(pin_state >= -1 && pin_state <= 1){
+
+          for(int8_t i = 0; i < (int8_t)sizeof(sensitive_pins); i++)
+          {
+            if (sensitive_pins[i] == pin_number)
+            {
+              pin_number = -1;
+              break;
+            }
+          }
+
+          if (pin_number > -1)
+          {
+            st_synchronize();
+
+            pinMode(pin_number, INPUT);
+
+            int target;
+            switch(pin_state){
+            case 1:
+              target = HIGH;
+              break;
+
+            case 0:
+              target = LOW;
+              break;
+
+            case -1:
+              target = !digitalRead(pin_number);
+              break;
+            }
+
+            while(digitalRead(pin_number) != target){
+              manage_heater();
+              manage_inactivity();
+              lcd_update();
+            }
+          }
+        }
+      }
+    }
+    break;	
 
     #if NUM_SERVOS > 0
     case 280: // M280 - set servo position absolute. P: servo index, S: angle or microseconds