Fixed authentication, added examples.

Authentication has been fixed and is now working. The issue was with the UID, as we only want the first 4 bytes of the backdate (the UID) appended when authenticating.

I've also removed the code in the bottom of the file MFRC522.py, as it should be easy to import it to whatever project you're doing. Instead I've added some proper examples showing how to read, write, and dump data. 

Some comments have also been added here and there for better documenting what is happening. A function called StopCrypto1() has also been added for stopping authentication so that you can scan another card.