add disconnect method to fona

Author: brentru

src/AdafruitIO_FONA.h

@@ -123,6 +123,18 @@ class AdafruitIO_FONA : public AdafruitIO {
 
     _status = AIO_NET_DISCONNECTED;
   }
+
+  /**************************************************************************/
+  /*!
+      @brief  Disconnects from Adafruit IO and the cellular network.
+  */
+  /**************************************************************************/
+  void _disconnect() {
+      if (!_fona->enableGPRS(false)) {
+          AIO_DEBUG_PRINTLN("Failed to turn off GPRS.");
+      }
+      _status = AIO_NET_DISCONNECTED;
+  }
 };
 
 #endif // ADAFRUITIO_FONA_H