Added writeFast() cmd - fill FIFO or return 0

- WriteFast allows the FIFO buffers to be filled up before returning 0
- The current payload will be dropped, and the previous payload resent
- The user should just keep trying to send the same data if a 0 is
received

RF24.h

@@ -225,6 +225,7 @@ public:
 	void txStandBy();
 	bool writeBlocking( const void* buf, uint8_t len );
 	void reUseTX();
+    bool writeFast( const void* buf, uint8_t len ); //Fills FIFO buffer, uses packet re-use function of the chip and returns 0 if packet failed and re-sent
 
   /**
    * @name Primary public interface