add demo for showing incoming call

examples/GPRS_Show_Incoming_Call/GPRS_Show_Incoming_Call.ino

@@ -0,0 +1,51 @@
+/*
+SIM900 Serial Debug 
+
+This sketch is used to communicate with SIM900 with AT commands.
+
+create on 2015/05/14, version: 1.0
+by lawliet.zou(lawliet.zou@gmail.com)
+*/
+#include "GPRS_Shield_Arduino.h"
+#include <Wire.h>
+
+const int PIN_TX = 7;
+const int PIN_RX = 8;
+const int BAUDRATE = 9600;
+
+GPRS gprs(PIN_TX,PIN_RX,BAUDRATE);//RX,TX,PWR,BaudRate
+char in_number[16] = {0};
+
+void setup(){
+  gprs.checkPowerUp();
+  Serial.begin(9600);  
+}
+
+void loop(){
+//  gprs.AT_Bypass();
+
+  /**
+   * Result code: 
+   * 0: ready
+   * 2: unknown
+   * 3: ringing
+   * 4: call in progress
+  */
+  bool is_active;
+  char in_number[16] = {0};
+
+  is_active = gprs.isCallActive(in_number);  
+
+  Serial.print("Active code : ");
+  Serial.println(is_active);
+  if(is_active) { 
+    Serial.print("Phone call in: ");
+    Serial.println(in_number);
+  } else {
+    Serial.println("No phone call in.");
+  }
+
+  delay(1000);  
+}
+
+