De-Activate features by default with RPi

Since RPi behaves more like a PC than Arduino, users should be able to
execute various programs without powering down the radio module, where
an Arduino needs to be re-programmed between code changes, so a full
power-down is not such a dramatic step.

Thanks to sej7278 (GitHub) see:
https://github.com/TMRh20/RF24Network/issues/2

RPi/RF24/RF24.cpp

@@ -474,6 +474,10 @@ bool RF24::begin(void)
   // Initialize CRC and request 2-byte (16bit) CRC
   setCRCLength( RF24_CRC_16 ) ;
 
+  toggle_features();
+  write_register(FEATURE,0 );
+  write_register(DYNPD,0);
+  
   // Reset current status
   // Notice reset and flush is the last thing we do
   write_register(STATUS,_BV(RX_DR) | _BV(TX_DS) | _BV(MAX_RT) );