Fixed bug in VerifySignature; other improvements

- Fixed the VerifySignature function; it always returned false because
it wasn't incrementing a pointer
- Added debugging feature to show EEPROM operations but not execute
them, if enabled
- VerifySignature won't be called twice when Begin is called
- All functions that access the EEPROM were moved to the .cpp module so
the header file doesn't need to include avr/eeprom.h anymore, and
neither does the sketch
- Instead of overriding the cast operator, the template class now gives
public access to the data member
- The assignment operator was removed. That way, all EEPROM operations
are explicit. If implicit operations are needed, it's always possible to
subclass the template class.