Added powerDown().

RF24.cpp

@@ -333,6 +333,13 @@ void RF24::stopListening(void)
 
 /******************************************************************/
 
+void RF24::powerDown(void)
+{
+  write_register(CONFIG,0);
+}
+
+/******************************************************************/
+
 boolean RF24::write( const void* buf, uint8_t len )
 {
   boolean result = false;

RF24.h

@@ -223,6 +223,14 @@ public:
    */
   void stopListening(void);
 
+  /**
+   * Enter low-power mode
+   *
+   * To return to normal power mode, either write() some data or
+   * startListening().
+   */
+  void powerDown(void);
+
   /**
    * Write to the open writing pipe
    *