Refactored and commented Default Keys example

Updated the `rfid_default_keys.ino` example sketch/program:

* Changed the description, to show that it is an example for the MFRC522 library
* Added link to the GitHub based project for the MFRC522 library
* Expanded the description of the example a little
* Updated the pin layout based on #48 updated README
* Put known default keys into an array (rather than code), which IMO makes the code easier to follow and allow easier expansion of the list whenever new keys are added to the list
* Added a helper routine to output a byte array to Serial in hex values; simplifies other code as well