Update RFID.h

Updated with SelectTag function. Needed for reading/writing blocks on a card. 
Reading goes like this:
1. isCard
2. readCardSerial or anticoll
3. selectTag
4. auth
5. read or write
Upadte RFID.cpp with selectTag

Upadte RFID.cpp with selectTag
Merge pull request #1 from martindamp/patch-1

Patch 1