Fix RF24NetworkHeader::toString() and remove commented out line

- Fix RF24NetworkHeader::toString so it can work with non-print-able chars

- Remove [commented out] line that has been moved a few lines below:
    // First, stop listening so we can talk.
    //radio.stopListening();
Merge pull request #30 from flavio-fernandes/master

Fix RF24NetworkHeader::toString() and remove commented out line