Add optional write timeout/failure handling

Per issue #5 by sven337
- Add optional failure handling/timeout
- Un-comment #define FAILURE_HANDLING in RF24_config.h to enable
- Add radio.errorDetected variable - indicates if an error/timeout was
detected
example:
if(radio.failureDetected){
delay(1000);
radio.begin();
radio.failureDetected = 0;
radio.openWritingPipe(addresses[1]);
radio.openReadingPipe(1,addresses[0]);
report_failure(); //blink leds, send a message, etc.
}

Additional:
- removed unused wide_band boolean
- remove rx buffer flushes for RPi also