added support to retrieve the device serial number

usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/CommonUsbSerialPort.java

@@ -79,6 +79,15 @@ abstract class CommonUsbSerialPort implements UsbSerialPort {
     public int getPortNumber() {
         return mPortNumber;
     }
+    
+    /**
+     * Returns the device serial number
+     *  @return serial number
+     */
+    @Override
+    public String getSerial() {
+        return mConnection.getSerial();
+    }
 
     /**
      * Sets the size of the internal buffer used to exchange data with the USB

usbSerialForAndroid/src/main/java/com/hoho/android/usbserial/driver/UsbSerialPort.java

@@ -90,6 +90,11 @@ public interface UsbSerialPort {
      * Port number within driver.
      */
     public int getPortNumber();
+    
+    /**
+     * The serial number of the underlying UsbDeviceConnection, or {@code null}.
+     */
+    public String getSerial();
 
     /**
      * Opens and initializes the port. Upon success, caller must ensure that