Added isValid() method to RF24.h as requested by andreij in http://forum.mysensors.org/topic/6/update-rf24-library-to-latest-version#12

Author: bblacey

libraries/RF24/RF24.h

@@ -673,6 +673,14 @@ class RF24
    */
   bool testRPD(void) ;
 
+  /**
+   * Test whether this is a real radio, or a mock shim for
+   * debugging.  Setting either pin to 0xff is the way to
+   * indicate that this is not a real radio.
+   *
+   * @return true if this is a legitimate radio 
+   */
+  bool isValid() { return ce_pin != 0xff && csn_pin != 0xff; } 
 
   /**
    * Calculate the maximum timeout in us based on current hardware